Allow static files in Collections

Allow StaticFiles in Collections alongside Documents. Attempts to fix #2555, #2598. Can also be modified a bit to accommodate #2592. Builds on #2603.

Please test!
Hold on - I just remembered that in 7eebb8248e54a2d82c17f41aa2a35f84e3021e46 I switched from using StaticFiles to Documents as you suggested. Which path should we take?