In order to lookup a Project or Namespace by path, we prefer an exact
match (case-sensitive) but in absence of that, we'd also take a
case-insensitive match.

The case-insensitive matching with preference for the exact match is a
bit more involved in SQL as the exact lookup. Yet, the majority of cases
will be an exact match. The thinking here is that we can optimize the
lookup by performing an exact match first and only if there is no
result, we perform the case-insensitive lookup.

Data for GitLab.com:
* We have about 15M records in routes table
* About 2,500 routes exist where there's more than one record
  with the same `lower(path)`

It is possible for a user to craft requests that would always trigger
the 2-step search (e.g. we have a route for `/foo/bar`, the request is
always for `/FOO/bar`). In this case, the change at hand is not
beneficial as it would run an additional query.

However, based on the data, it is highly likely that the vast majority
of requests can be satisfied with an exact match only.

The context for this change is
https://gitlab.com/gitlab-org/gitlab-ce/issues/64590#note_208156463.

Instead of setting the name of the namespace to the user's username,
set it to the user's name.

This is more consistent with how we name the routes:
The route-name of a namespace is the human name of the routable. In
the case of a user-namespace, this is the owner's name.

When we change a user's name (both on create and update), we now also
update the namespace-name to the user's name. This will make sure that
if we also correctly update all the nested routes.

Adds `# frozen_string_literal: true` to spec/models ruby files

This adds a database migration that creates routes for any projects and
namespaces that don't already have one. We also remove the runtime code
for dynamically creating routes, as this is no longer necessary.

In GitLab EE, a GitLab instance can be read-only (e.g. when it's a Geo
secondary node). But in GitLab CE it also might be useful to have the
"read-only" idea around. So port it back to GitLab CE.

Also having the principle of read-only in GitLab CE would hopefully
lead to less errors introduced, doing write operations when there
aren't allowed for read-only calls.

Closes gitlab-org/gitlab-ce#37534.

This commit introduces the usage of Common Table Expressions (CTEs) to
efficiently retrieve nested group hierarchies, without having to rely on
the "routes" table (which is an _incredibly_ inefficient way of getting
the data). This requires a patch to ActiveRecord (found in the added
initializer) to work properly as ActiveRecord doesn't support WITH
statements properly out of the box.

Unfortunately MySQL provides no efficient way of getting nested groups.
For example, the old routes setup could easily take 5-10 seconds
depending on the amount of "routes" in a database. Providing vastly
different logic for both MySQL and PostgreSQL will negatively impact the
development process. Because of this the various nested groups related
methods return empty relations when used in combination with MySQL.

For project authorizations the logic is split up into two classes:

* Gitlab::ProjectAuthorizations::WithNestedGroups
* Gitlab::ProjectAuthorizations::WithoutNestedGroups

Both classes get the fresh project authorizations (= as they should be
in the "project_authorizations" table), including nested groups if
PostgreSQL is used. The logic of these two classes is quite different
apart from their public interface. This complicates development a bit,
but unfortunately there is no way around this.

This commit also introduces Gitlab::GroupHierarchy. This class can be
used to get the ancestors and descendants of a base relation, or both by
using a UNION. This in turn is used by methods such as:

* Namespace#ancestors
* Namespace#descendants
* User#all_expanded_groups

Again this class relies on CTEs and thus only works on PostgreSQL. The
Namespace methods will return an empty relation when MySQL is used,
while User#all_expanded_groups will return only the groups a user is a
direct member of.

Performance wise the impact is quite large. For example, on GitLab.com
Namespace#descendants used to take around 580 ms to retrieve data for a
particular user. Using CTEs we are able to reduce this down to roughly 1
millisecond, returning the exact same data.

== On The Fly Refreshing

Refreshing of authorizations on the fly (= when
users.authorized_projects_populated was not set) is removed with this
commit. This simplifies the code, and ensures any queries used for
authorizations are not mutated because they are executed in a Rails
scope (e.g. Project.visible_to_user).

This commit includes a migration to schedule refreshing authorizations
for all users, ensuring all of them have their authorizations in place.
Said migration schedules users in batches of 5000, with 5 minutes
between every batch to smear the load around a bit.

== Spec Changes

This commit also introduces some changes to various specs. For example,
some specs for ProjectTeam assumed that creating a personal project
would _not_ lead to the owner having access, which is incorrect. Because
we also no longer refresh authorizations on the fly for new users some
code had to be added to the "empty_project" factory. This chunk of code
ensures that the owner's permissions are refreshed after creating the
project, something that is normally done in Projects::CreateService.

We see in #27387 that a call to `polymorphic_path` will cause duplicate
SELECT route calls for each merge request in a milestone. This happens
because calling `project.namespace.becomes(Namespace)` will instantiate
a new instance of a Namespace for each merge request, which causes a N+1
query on the routes table. This change caches the state of the route by
the specific class and ID, which dramatically eliminates duplicate work.

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

* add parent_id field to namespaces table to store relation with nested groups
* create routes table to keep information about full path of every group and project
* project/group lookup by full path from routes table
Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>