metrics: most frequent questions/topics

Currently, our monthly summaries show a tag cloud outlining which topics/questions come up most frequently. Dan pointed out that it would be able to see these topics on a "this month" vs "all-time" basis, like we're now doing for feature requests.

No files, tests, or entry points are named. Start by locating the monthly summary and tag-cloud generation, then compare how feature requests implement “this month” versus “all-time”; done means the topic metrics support both time ranges in the summaries.

Written by the indexing model from the issue text.