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!
