Add way to move multiple (or all) files from incoming/ to unprocessed/

[DavidKinder]

If there are a bunch of files in incoming/, moving them one at a time to unprocessed/ is a bit laborious. What actually happens in practice is that there's some renaming and/or zipping to get the files into the right state, and then they all need moving to unprocessed/. It would be nice if there were a way to move all of them in one go.

[erkyrath]

Multi-select should be doable, although it will take a bunch more form code.