Add overview of dispatch states to DispatchStats.

This provides an overview of how many client wikis are in which
dispatch state (complete, locked, fresh, or pending).

Change-Id: Id1ee5634764589b7155989dab78dc73bb9c59b7d
Merge commit 'refs/changes/64/58464/3' of ssh://gerrit.wikimedia.org:29418/mediawiki/extensions/Wikibase