Document access key permissions: gitlab

[applejag]

When importing from either provider, you need an access key. The access this key needs has to be documented, as it's an obvious question that arises.

Todo:

• Look at the code (https://github.com/iver-wharf/wharf-provider-gitlab), see what it's doing, and coming up with the assumed permissions
• Do some manual testing to verify with a key that only has those permissions
  • importing new project
  • refreshing the project
• document the required permissions.

We should make it possible to update the credentials for a project in the future when we need it. Such as when we need to set commit statuses, we then need permission for that. But let's follow the "principle of least privilage"