Custom font size
Fixes #566.
This adds a user setting to change the font size of Tildes. As well as a fix for the Search button, which turned out to be a Firefox-specific issue (as far as I could tell, it works fine on Chromium and Safari). I had some questions/uncertainties about specific things so I've made comments in the diff about them.
For the options I decided to go with presets as I mentioned Discord does in #566, having all 16 through 32 in the list was a bit overwhelming.
At one point I thought it might be useful to add the same cookie-based functionality from the theme one, so you have an account default and maybe a different one on your device. However, because we directly assign the cookie in the theme setting we'd have to change it so we can properly parse cookies and only change a specific one. Which I thought was a little out of scope for me for now.