Specify base controller for Doorkeeper

What does this MR do?

This fixes a failure discovered by QA: gitlab-qa#198 (comment 60735266)

Logging in using Oauth would fail on a secondary.

By setting the base_controller for doorkeeper we make sure the ApplicationController#can? helper method is available to the views.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes gitlab-qa#198 (comment 60735266)