Custom Right-Hand Side Background Image

[alice-debra]

Allow platform administrators to upload a custom image that replaces the entire right-hand side panel of the login page, while keeping the Filigran marking in place.

[SamuelHassine]

@alice-debra @esrevi, finding the right image can be difficult, I propose also that the user can choose plain color or gradiant to just having full colored panel or gradiant panel corresponding to its theme in addition to an image = so image or plain color or gradiant color.