theme statistics (bug 565813)

**Notes:**

- Daily cronjob saves Persona.popularity into stats.models.ThemeUserCount with addon/date/count.
- Adds an indexing task to index_stats to save the daily theme user counts into ElasticSearch.
- Creates a link to the statistics dashboard from the theme dashboard and theme edit page.
- Theme statistics dashboard only contains an active daily user chart (usage) with no breakdowns.

[!Dashboard Screenshot](http://i.imgur.com/lfq3Ot2.png)