Add FileStore.soft_max_size and FileStore.max_size

soft_max_size doesn't do anything yet; we'll handle it in debusine-admin vacuum_storage soon.

max_size is checked each time we look for a file store that can accept an upload, so it needs to be efficient, and summing the size of all the files in the store is a bit too slow on large stores (I timed it at around 390 ms on debusine.debian.net). I therefore added FileStore.total_size, maintained by a trigger, allowing us to check against that quickly.

Fixes: #682 (closed)