[TASK] Cleanup BackendController.php (part 1)

The backend controller has some leftover
code. First, the main output is moved
to a standalone fluid view.

Additionally, the UserMenu.js is not
needed anymore, as Bootstrap Dropdown
is taking care of its logic.

Some minor code logic cleanup is also
added, as well as a @todo note.

Releases: master
Resolves: #63145
Change-Id: I9323a036a160561a165ede22a3e707bbe4a540ae
Reviewed-on: http://review.typo3.org/34469
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>