OAuth for API access

In reference to this micro-code-review app, it needs a way to authenticate and connect to the real GitLab API.

Benefits

• The MR displayed is currently totally arbitrary. With an OAuth connection, a user could load any MR they have access to.
• We don't need a special back end, the API should be enough

Complications

• We'll probably have to create at least one server-side function to negotiate the OAuth connection.

Future upgrade paths

• Having access to the API means we could implement many standard features. However, in the interest of the "Micro" concept here, we should limit our use very strictly.