bug 831444: remove unused gallery app

Since we had a contributor issue a PR to an unused app in #1032, and since I have most of the work laying around unmerged in [a 3-month-old branch](https://github.com/lmorchard/kuma/commits/purge-apps-831444), I figured I might just as well start cherry-picking and submitting pulls to start excising these apps.

I've got 14 commits in that branch. Rather than dump them all in at once, I'm going to trickle them in one PR at a time: Let tests run, make sure nothing obvious is broken, push to production, repeat until the cleanup effort is done.