Create child folder objects using parent context for real

We activate the user for the context using the root folder
context as there are times where the active user is not
matching with the one stored in the application context
and SOGo object is storing cached data with the wrong user
leading to create folders in wrong mailboxes, etc.

As this application is single-threaded, no problems are expected.
Indeed, the same code is available at getting the root folder (ie INBOX).