Fixes #26051.
Signed-off-by: NRémy Coutable <remy@rymai.me>

- Move the `Oauth2::AccessTokenValidationService` class to
  `AccessTokenValidationService`, since it is now being used for
  personal access token validation as well.

- Each API endpoint declares the scopes it accepts (if any). Currently,
  the top level API module declares the `api` scope, and the `Users` API
  module declares the `read_user` scope (for GET requests).

- Move the `find_user_by_private_token` from the API `Helpers` module to
  the `APIGuard` module, to avoid littering `Helpers` with more
  auth-related methods to support `find_user_by_private_token`

Adding the necessary API for the new /snippets Restful resource
added with this commit. Added a new Grape class `Snippets`, as
well as a `PersonalSnippet` entity.

Issue: #20042
Merge-Request: !6373
Signed-off-by: NGuyzmo <guyzmo+gitlab+pub@m0g.net>

The /licenses, /gitignores and /gitlab_ci_ymls endpoints are now also
available under a new /templates namespace. Old endpoints will be
deprecated when GitLab 9.0.0 is released.

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

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

  - Includes documentation and tests

Use NotificationSetting::EMAIL_EVENTS for params

Closes #21043

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

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

Also a couple of minor edits for this branch are included

Basically:

    GET /api/projects/:id/artifacts/:ref_name/:build_name

Also added tests for it.