In this PR:
1. Refactor guards to avoid duplicated queries: WorkspaceAuthGuard and
UserAuthGuard only check for existence of workspace and user in the
request without querying the database
Closes#4295
Note: for the sake of an easier code review, I did not rename/move some
files and added "todo" comments instead so Github is able to match those
files with their previous version.
Charles Bochet