[FEATURE] Introduce hook in BackendUserAuthentication::getDefaultUploadFolder

A hook is introduced to manipulate the default upload folder of FAL
inline upload and RTE text fields.

Change-Id: Ib497b02c90d3217062d2abb55eef693e5c445c46
Resolves: #68895
Releases: master
Reviewed-on: http://review.typo3.org/42470


Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>