Changes the two settings pages that show the size of a site (from the
main menu, these are 'App settings / Space usage' and 'Preferences')
so that they:
a) do not show the number of cache entries (not very informative for
end users)
b) show an estimate for 'space usage' that includes the total size
of data in ws_cache
The space usage estimate is not perfect (it doesn't include the
per-row overhead in the ws_cache table, or other cached data that
can be deleted such as calendar events or user data) but it will
probably be good enough, and I don't think the 'number of entries'
things was helpful to anyone.
