Branding: background customization

[privsec]

As requested here, allow for a css responsive background image as primary background display option and use a color as a secondary option to display in case the image can’t load for any reason.

[girish]

@privsec This is available at a user level - https://docs.cloudron.io/profile/#background-image

[privsec]

@girish Can this be set at an organizational level?