Personal access token api and impersonation token

What does this MR do?

It adds the impersonation token and the management of the personal access tokens through api.

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

Maybe he should double check in lib/gitlab/auth.rb. Thanks in advance @smcgivern :)

Does this MR meet the acceptance criteria?

• Changelog entry added
• 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?

Please see https://gitlab.com/gitlab-org/gitlab-ce/issues/25367.

Screenshot

/profile/personal_access_tokens

before

after

/admin/users/:username

before

after

without any tokens

after creating one token

with every type of token

after revoking one token

Some notes:

• I change the expires_at field from datetime to date because I see that every expiration date is a date.
• I included the concern expirable in personal_access_token.