[FEATURE] Add configurable footnote to login screen

We are introducing a new option to provide an optional footnote for the 
login screen. The footnote can provide for example legal information for
the login screen image or a descriptive text. This option is now 
available in the backend extension configuration.

Resolves: #82303
Releases: master
Change-Id: I14f90ecfa8eee538234dcc9542576f6ab9f1d774
Reviewed-on: https://review.typo3.org/53918


Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Riccardo De Contardi <erredeco@gmail.com>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Joerg Kummer <typo3@enobe.de>
Tested-by: Joerg Kummer <typo3@enobe.de>
Reviewed-by: Benjamin Kott <benjamin.kott@outlook.com>
Tested-by: Benjamin Kott <benjamin.kott@outlook.com>