Previously we would bypass touching `Topic.updated_at` for whispers and post
recovery / deletions.
This meant that certain types of caching can not be done where we rely on
this information for cache accuracy.
For example if we know we have zero unread topics as of yesterday and whisper
is made I need to bump this date so the cache remains accurate
This is only half of a larger change but provides the groundwork.
Confirmed none of our serializers leak out Topic.updated_at so this is safe
spot for this info
At the moment edits still do not change this but it is not relevant for the
unread cache.
This commit also cleans up some specs to use the new `eq_time` matcher for
millisecond fidelity comparison of times
Previously `freeze_time` would fudge this which is not that clean.
