Rename methods in the LoginHelpers module

- [x] Rename `login_as` to `gitlab_sign_in`
- [x] Rename `login_with` to `gitlab_sign_in_with`
- [x] Rename `login_via` to `gitlab_sign_in_via`
- [x] Rename `logout` to `gitlab_sign_out`
- [x] Rename `logout_direct` to `gitlab_sign_out_direct`
- [x] Make `gitlab_login_with` (née `login_with`) private
- [x] Make `mock_auth_hash` private