oauth: Re-enable OAuth and switch to read_api for GitLab

Created by: ryanslade

We need to request full api access for GitLab even if the user only requires public code access since GitLab doesn't appear to offer scopes with the granularity we need. You either get access to everything for a user or nothing.

We've reverted to asking for full api access again and re-enabled OAuth for user added code, but we're now only requesting read access whereas before we requested read/write access.