Save uploaded background image in meme generate

[Starefossen]

Description
The ability to save uploaded background images in the meme generator.

Motivation
This will allow for greater reuse of background images across marketing members.

Additional context
It is a great tool, this will only make it better.

[Starefossen]

Also it should be an option to just use the logo mark (without text).
It should also be an option to add a footer with the url of the conference.