Various statistics about revisions at /differential/stats/revisions/

Summary:
Show some statistics, like number of revisions, number of
revisions per week, lines per revision, etc. for phrivolous amusement.

Test Plan:

• Went to /differential/stats/revisions/

Numbers seem right

• Clicked 'Accepted'

Again

• Changed to another user with long history

Load time was not too long though delay noticeable

• Clicked 'Requested changes to'

User was preserved, looks good

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1643