Add box for custom css snippets

@nicol this adds a box for freeform custom css snippets.

I think it's probably quite inefficient performance wise if you add any significant amount of css here (because atm it's stored in the database, and basically has to be fetched on every request).

There are definitely ways to improve the performance in the long run, but they require a bit of building out / thinking about.

For now I would say it's probably fine if you have 1 or 2 blocks in there, but just to be mindful of discouraging folks from writing whole themes in that box.