An upcoming update to rubocop-gitlab-security added additional
violations.

- To represent an authorization scope, such as `api` or `read_user`
- This is a better abstraction than the hash we were previously using.

- Use a struct for scopes, so we can call `scope.if` instead of `scope[:if]`

- Refactor the "remove scopes whose :if condition returns false" logic to use a
  `select` rather than a `reject`.

1. Get the spec for `lib/gitlab/auth.rb` passing.

  - Make the `request` argument to `AccessTokenValidationService` optional -
  `auth.rb` doesn't need to pass in a request.

  - Pass in scopes in the format `[{ name: 'api' }]` rather than `['api']`, which
  is what `AccessTokenValidationService` now expects.

2. Get the spec for `API::V3::Users` passing

2. Get the spec for `AccessTokenValidationService` passing

- They are not included automatically since `API::Users` does not inherit from
  `API::API`, as I initially assumed.

- Scopes declared in `API::API` are considered global (to the API), and need to
  be included in all cases.

- Scope declarations of the form:

    allow_access_with_scope :read_user, if: -> (request) { request.get? }

  will only apply for `GET` requests

- Add a negative test to a `POST` endpoint in the `users` API to test this. Also
  test for this case in the `AccessTokenValidationService` unit tests.

- Declaring an endpoint's scopes in a `before` block has proved to be
  unreliable. For example, if we're accessing the `API::Users` endpoint - code
  in a `before` block in `API::API` wouldn't be able to see the scopes set in
  `API::Users` since the `API::API` `before` block runs first.

- This commit moves these declarations to the class level, since they don't need
  to change once set.