Add logging of search quality information

For now, this only includes course id, search terms, and result count.
Information is logged both to the standard logger for near-term analysis
and the event tracker for the longer term. Result count logging requires
commit 02466b1 of cs_comments_service (otherwise it will be null).
Merge pull request #3728 from edx/gprice/forum-search-logging

Add logging of search quality information