[FEATURE] Avatar for backend users

Extends be_users with avatar image.

* Avatars can be uploaded by backend users via EXT:setup.
* TCA of backend users is extended with image column.
* Added new ViewHelper to show the avatar everywhere in the backend.

Resolves: #48947
Resolves: #57515
Releases: master
Change-Id: I4b0e4b5d31d04fd6dfe1f7c7f52b71762a8e016a
Reviewed-on: http://review.typo3.org/24132


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Benjamin Mack <benni@typo3.org>
Reviewed-by: Benjamin Kott <info@bk2k.info>
Tested-by: Benjamin Kott <info@bk2k.info>
Tested-by: Benjamin Mack <benni@typo3.org>