TokenAuthenticatable allows non-unique tokens

What does this MR do?

Allows non-unique tokens. Avoids a database lookup when saving a token that would also have required a DB index.

Related

CE changes for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8016 These changes were originally in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22420, but that was re-written after TokenAuthenticatable changes and no longer needed a workaround for length.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.