Change admin bro "upload banner for QF" round - storing 3 versions (tablet, mobile, desktop) for each qf round

[laurenluz]

right now, we have this in admin bro, where the admin is supposed to upload each sponsor image separate, but there are constraints that don't meet our requirements (a limit of 6 sponsors, 1 size, standard placement).

To accomodate rounds with varying levels of sponsors and different sponsor positions, we should change this to allow the admin to upload 3 version of the banner

• Mobile
• Tablet
• Desktop

And those images should be saved in the background to correspond to each qf round number... so that they can be preserved and used in the "archived qf round" views.

@mosaeedi please specify the sizes & specs for each image that would be most appropriate for these responsive versions.

cc @Meriem-B @koday1 @maryjaf