Changes include:

- Ensure Member.add_user is not called directly when not necessary
- New GroupMember.add_users_to_group to have the same abstraction level as for Project
- Refactor Member.add_user to take a source instead of an array of members
- Fix Rubocop offenses
- Always use Project#add_user instead of project.team.add_user
- Factorize users addition as members in Member.add_users_to_source
- Make access_level a keyword argument in GroupMember.add_users_to_group and ProjectMember.add_users_to_projects
- Destroy any requester before adding them as a member
- Improve the way we handle access requesters in Member.add_user
  Instead of removing the requester and creating a new member,
  we now simply accepts their access request. This way, they will
  receive a "access request granted" email.
- Fix error that was previously silently ignored
- Stop raising when access level is invalid in Member, let Rails validation do their work
Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NRémy Coutable <remy@rymai.me>

The performance was greatly improved by removing two N+1 queries issues
for each endpoint.

For comparison:

1. `GET /projects/:id/members`

With two N+1 queries issues (one was already fxed in the following
snippet):

```
  ProjectMember Load (0.2ms)  SELECT "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND "members"."requested_at" IS
NULL  ORDER BY "members"."id" DESC  [["source_type", "Project"],
["source_id", 1], ["source_type", "Project"]]
  User Load (0.5ms)  SELECT "users".* FROM "users" WHERE "users"."id" IN
(5, 22, 16, 13)  ORDER BY "users"."id" DESC
  ActiveRecord::SchemaMigration Load (0.2ms)  SELECT
"schema_migrations".* FROM "schema_migrations"
  ProjectMember Load (0.3ms)  SELECT  "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND "members"."requested_at" IS
NULL AND "members"."user_id" = $4  ORDER BY "members"."id" DESC LIMIT 1
[["source_type", "Project"], ["source_id", 1], ["source_type",
"Project"], ["user_id", 5]]
  ProjectMember Load (0.3ms)  SELECT  "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND "members"."requested_at" IS
NULL AND "members"."user_id" = $4  ORDER BY "members"."id" DESC LIMIT 1
[["source_type", "Project"], ["source_id", 1], ["source_type",
"Project"], ["user_id", 22]]
  ProjectMember Load (0.3ms)  SELECT  "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND "members"."requested_at" IS
NULL AND "members"."user_id" = $4  ORDER BY "members"."id" DESC LIMIT 1
[["source_type", "Project"], ["source_id", 1], ["source_type",
"Project"], ["user_id", 16]]
  ProjectMember Load (0.3ms)  SELECT  "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND "members"."requested_at" IS
NULL AND "members"."user_id" = $4  ORDER BY "members"."id" DESC LIMIT 1
[["source_type", "Project"], ["source_id", 1], ["source_type",
"Project"], ["user_id", 13]]
```

Without the N+1 queries issues:

```
  ProjectMember Load (0.3ms)  SELECT  "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND "members"."requested_at" IS
NULL  ORDER BY "members"."id" DESC LIMIT 20 OFFSET 0  [["source_type",
"Project"], ["source_id", 1], ["source_type", "Project"]]
  User Load (0.5ms)  SELECT "users".* FROM "users" WHERE "users"."id" IN
(5, 22, 16, 13)  ORDER BY "users"."id" DESC
```

2. `GET /projects/:id/access_requests`

With two N+1 queries issues:

```
  ProjectMember Load (0.3ms)  SELECT "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND ("members"."requested_at" IS
NOT NULL)  ORDER BY "members"."id" DESC  [["source_type", "Project"],
["source_id", 8], ["source_type", "Project"]]
  User Load (0.1ms)  SELECT  "users".* FROM "users" WHERE "users"."id" =
$1  ORDER BY "users"."id" DESC LIMIT 1  [["id", 24]]
  User Load (0.1ms)  SELECT  "users".* FROM "users" WHERE "users"."id" =
$1  ORDER BY "users"."id" DESC LIMIT 1  [["id", 23]]
  ActiveRecord::SchemaMigration Load (0.2ms)  SELECT
"schema_migrations".* FROM "schema_migrations"
  ProjectMember Load (0.1ms)  SELECT  "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND ("members"."requested_at" IS
NOT NULL) AND "members"."user_id" = $4  ORDER BY "members"."id" DESC
LIMIT 1  [["source_type", "Project"], ["source_id", 8], ["source_type",
"Project"], ["user_id", 24]]
  ProjectMember Load (0.2ms)  SELECT  "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND ("members"."requested_at" IS
NOT NULL) AND "members"."user_id" = $4  ORDER BY "members"."id" DESC
LIMIT 1  [["source_type", "Project"], ["source_id", 8], ["source_type",
"Project"], ["user_id", 23]]
```

Without the N+1 queries issues:

```
  ProjectMember Load (0.3ms)  SELECT  "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND ("members"."requested_at" IS
NOT NULL)  ORDER BY "members"."id" DESC LIMIT 20 OFFSET 0
[["source_type", "Project"], ["source_id", 8], ["source_type",
"Project"]]
  User Load (0.5ms)  SELECT "users".* FROM "users" WHERE "users"."id" IN
(24, 23)  ORDER BY "users"."id" DESC
```
Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NRémy Coutable <remy@rymai.me>

Also, mutualize AccessRequests and Members endpoints for Group &
Project.
New API documentation for the AccessRequests endpoints.
Signed-off-by: NRémy Coutable <remy@rymai.me>