Relates to #86. If you are viewing a filtered page e.g. https://fivethirtyeight.datasettes.com/fivethirtyeight-2628db9/bob-ross%2Felements-by-episode?CLOUDS=1 we should show the count of matching rows.

Since this could be an expensive operation, we will run it with a strict time limit (maybe 50ms). If the time limit is exceeded we will display "many" instead, perhaps? Maybe even link to a count(*) query that would get the full 1000ms time limit which the user can click on if they like (that could even Ajax-in the result).