UI to manage API users

www.agileplannerapp.com/boards/173808/cards/4993

introducing a UI to:
1. list, create, edit, suspend API users
2. create, revoke and regenerate tokens for API users

see commit messages for additional information.
I looked at a couple other places, they only show a list of errors without the <h4>. so did that in https://github.com/alphagov/signonotron2/commit/d0c8f5fe2478d99a08926fecf6fbafcbd4c24120