mw.inspect: add report for mw.loader.store

Adds a 'store' report to mw.inspect, which outputs:
* Whether localStorage module caching is enabled.
* Cache hit / miss counts.
* Number of items purged from the cache.
* Total size of the cache blob in localStorage.

Rather than duplicate the logic that converted a numeric byte count to a
human-readable format, I moved it to new helper function: humanSize.

Change-Id: I5b98322ba843f32e6a99829b4cf3d8fb0bc61514
(cherry picked from commit fbfca0d10f3cc5ad57c16f304bf8b3e6b3396db5)
Merge "mw.inspect: add report for mw.loader.store" into wmf/1.23wmf2