feat: add gitlab provider

[blumgart]

To create a new application for your user:

1. On the left sidebar, select your avatar.
2. Select Edit profile.
3. On the left sidebar, select Applications.
4. Select Add new application.
5. Enter a Name and Redirect URI.
6. Select OAuth 2 Scopes as defined in Authorized Applications.
7. In the Redirect URI, enter the URL where users are sent after they authorize with GitLab.
8. Select Save application. GitLab provides:
   • The OAuth 2 Client ID in the Application ID field.
   • The OAuth 2 Client Secret, accessible by selecting Copy in the Secret field.