Add the ability to customize the desktop background

[shane-tomlinson]

Mozilla China adds their own custom background image in the #stage element to give their FxA stack a local feel. At least one OAuth relier has asked for similar functionality. Let's start with the Mozilla China case and work from there, possibly A/B testing whether relier specific background images help conversion rates.

┆Issue is synchronized with this Jira Task
┆Issue Number: FXA-731

[shane-tomlinson]

cc @l-hedgehog

[AlbionaHoti]

Hi, @shane-tomlinson I am an outreachy applicant - Can I work on this issue?

[lmorchard]

Hmm, this issue is not on our "help wanted" list. It might need some more description of the work involved before it's ready to tackle. (At least, I'm not sure what the work entails)