Feature: #82303 - Add configurable footnote to login screen

See Issue #82303

Description

A new option to provide an optional footnote for the login screen has been introduced. 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.

Impact

If the footnote is configured the note will be displayed on the login screen. The background image of the login screen is now also shown on smaller devices.