- Previously, AccessTokenValidationService was a module, and all its  public
methods accepted a token. It makes sense to convert it to a class which accepts
a token during initialization.

- Also rename the `sufficient_scope?` method to `include_any_scope?`

- Based on feedback from @rymai

- Mainly whitespace changes.

- Require the migration adding the `scope` column to the
  `personal_access_tokens` table to have downtime, since API calls will
  fail if the new code is in place, but the migration hasn't run.

- Minor refactoring - load `@scopes` in a `before_action`, since we're
  doing it in three different places.

- 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`

The `guard_all!` method is never called, and `guard!` is not implemented. The
`doorkeeper_guard!` method is also never called, and is mostly the same as its
non-bang counterpart.

- Fix naming of API::CommitStatuses
- Ensure we use require_dependency instead of require
- Ensure the namespace is right in lib/api/api.rb, otherwise, we
  might require Grape::API::Helpers which defines the `#params` method.
  This is to avoid requiring a file multiple times and getting an "Already
  initialized constant" error.

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