API call to query the tasks using a particular rubric
Api call to query tasks using the latest revision of a rubric, or maybe all revisions, or a particular rev.
Or maybe a general key-value json return that we can get the rev info from ourselves.
Use this in the client "Modify Rubric" dialog to display "17 tasks are using this rubric", to prepare for future consequences of updating rubric (namely bulk tagging).
The reason to have more info than just the count in the API is that a more useful message is "17 tasks are using this rubric, with 15 of them already tagged attn". Rational: if the Rubric is undergoing revision anyway, its less work to modify it again before dealing with the fallout.