Two users editing fonts at the same time could cause conflicting writes to fonts.css

This should be a very obscure problem, as the scenario of multiple people adjusting fonts on any given site is pretty rare, and those people making changes at precisely the same time even rarer. It also might be better solved by the two people who are editing fonts talking to each other.

That said, while current use cases only involve administrators adjusting enabled fonts for the whole site, we may eventually have use cases for non-admin users adjusting enabled fonts for more specific contexts, which would make this a more common problem.

The suggested solution to this, writing to hashed file names, introduces some new problems, which I'd like to avoid if possible. It would make support more difficult, as we could no longer ask people about the contents of a specific file by a known name; we'd need to first explain how they can find the right file. And it also introduces the need for some sort of clean-up process to delete the old, unused hashes.