Note that these queries are a bit slow. I don't think these new subpages will be accessed anywhere near often enough for their ~2sec query time to be a big deal. But if we start getting into trouble with it (e.g. someone starts slamming us for fun), we can look into how how cache these values over time.
This is a bit more standard, and has the bonus of being compatible with Devise, which is using `flash[:notice]` and so its flashes were coming out unstyled, oops!
Some important little upgrades but mostly straightforward!
Note that there's still a known issue where item searches crash, I was hoping that this was a bug in Rails 4.2 that would be fixed on upgading to 5, but nope, oh well!
Also uhh I just got a bit silly and didn't actually mean to go all the way to 5.2 in one go, I had meant to start at 5.0… but tbh the 5.1 and 5.2 changes seem small, and this seems to be working, so. Yeah ok let's roll!
