[BUGFIX] Use absolute URL for avatars

Rendering avatar URLs which are not on a remote (gravatar etc)
is the only part which deals with references as in ".." preventing
the TYPO3 Backend entryscript to be placed somewhere else as in typo3/.

Resolves: #81025
Releases: master, 8.7
Change-Id: I9abc51532149fe90ba027608cfb6bad6ea66115d
Reviewed-on: https://review.typo3.org/52535


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Helmut Hummel <typo3@helhum.io>
Reviewed-by: Henrik Elsner <helsner@dfau.de>
Reviewed-by: Sascha Egerer <sascha@sascha-egerer.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>