Fetch map selector images based on screen resolution

[tsa96]

Game sibling of https://github.com/momentum-mod/website/issues/907. Map data from the backend (both paginated endpoint and flat file) include map images as arrays of

{
  id: <some uuid>,
  small: <that uuid>-small.jpg,
  medium: <that uuid>-medium.jpg,
  large: <that uuid>-large.jpg,
  xl: <that uuid>-xl.jpg (coming soon!)
}

with XL version coming soon. Map cache code should check the user's current resolution and use that to pick the type it downloads.

I'm working on map cache atm, so will probably be able to do this.