This adds a settings box to about:settings to allow users to limit the
disk cache size. This will override the default 5 GiB limit. We do not
automatically delete cache data if the new limit is suddenly less than
the used disk space; this will happen on the next request. This allows
multiple changes to the settings in a row without thrashing the cache.
In the future, we can add more toggles, such as disabling the disk
cache altogether.
