Compute lightweight theme popularity and movers (bug 1059964)

Fixes [bug 1059964](https://bugzilla.mozilla.org/show_bug.cgi?id=1059964)

This PR is based on #252
I don't know: there's a note in [bug 1059964](https://bugzilla.mozilla.org/show_bug.cgi?id=1059964) about resetting the movers field, but not the popularity field, and I thought it was just an omission.

What do you think?