Allow /stats to show all stats, provide raw filesize

#4846 

#### Changes

* Provides a new checkbox value for the `enabled_stats` variable: filesize bytes (for aggregating a formatted filesize across all installs, we must provide a raw byte size value on a per-site level)
* Provides a new query to find and expose the total filesize in bytes for all managed files.
* Changes the default parameter for "type" in the `"/stats"` menu item to be `"all"`, not `"websites"`