Search summary

[edemaine]

• Summary statistics at end of a search, like we have already for a thread ("Authors/@mentions of visible messages in this thread"). [Requires generalization of threadAuthors/threadMentions mechanism to readMessage]
• In search, ideally we'd also get a count of number of distinct authors too, like we currently do with threads. Combined with searching by root post (Search by root post #364), we could quickly compare SOLVED vs. OPEN solving students.
• Can we also get a list of people that don't appear in a search? I.e. detect the zeros. Superuser only?
• Alternatively, superusers could have an option to get the data (including zeros) as a CSV row or column with a consistent user sort order.

(moved from #364)