Send a confirmation email when the user adds a secondary email address. Utilizes the Devise `confirmable` capabilities. Issue #37385

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

New version of the gem returns 200 status code on delete with content
instead of 204 so we explicitly set status code to keep existing
behavior
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

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

- Rather than using an explicit check to turn off authentication for the
  `/users` endpoint, simply call `authenticate_non_get!`.

- All `GET` endpoints we wish to restrict already call
  `authenticated_as_admin!`, and so remain inacessible to anonymous users.

- This _does_ open up the `/users/:id` endpoint to anonymous access. It contains
  the same access check that `/users` users, and so is safe for use here.

- More context: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12445#note_34031323

- Use `GlobalPolicy` to authorize the users that a non-authenticated user can
  fetch from `/api/v4/users`. We allow access if the `Gitlab::VisibilityLevel::PUBLIC`
  visibility level is not restricted.

- Further, as before, `/api/v4/users` is only accessible to unauthenticated users if
  the `username` parameter is passed.

- Turn off `authenticate!` for the `/api/v4/users` endpoint by matching on the actual
  route + method, rather than the description.

- Change the type of `current_user` check in `UsersFinder` to be more
  compatible with EE.

- 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.

- The issue filtering frontend code needs access to this API for non-logged-in
  users + public projects. It uses the API to fetch information for a user by
  username.

- We don't authenticate this API anymore, but instead - if the `current_user` is
  not present:

  - Verify that the `username` parameter has been passed. This disallows an
    unauthenticated user from grabbing a list of all users on the instance. The
    `UsersFinder` class performs an exact match on the `username`, so we are
    guaranteed to get 0 or 1 users.
  - Verify that the resulting user (if any) is accessible to be viewed publicly
    by calling `can?(current_user, :read_user, user)`