[EE] Skip oAuth authorization for trusted applications

What does this MR do?

Enables admins to select an oAuth consumer application as trusted or not. Trusted applications doesn't have the Authorization click phase (this step is skipped).

Are there points in the code the reviewer needs to double check?

• Security!
• Will we provide such feature for self-hosted instances, or just GitLab.com?
• Will this also be available for CE?

Why was this MR needed?

We need a way to skip this phase for customers logging-in through GitLab.com on customers.gitlab.com, as this shows as an unnecessary extra step (since customers.gitlab.com is a trusted application).

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-com/customers-gitlab-com/issues/121