Ability to "tag" a run after the run is complete

[yymao]

A potential solution for #160 is for the landing page to display a set of runs that the catalog and test developers consider are suitable for general consumption.

To be able to do so, we need a new feature to allow test runners to "tag" a run after the run is complete. Currently DESCQA only allows test runners to tag at the time they initiate the run, but sometimes the run fails for unforeseen reasons and we would not want to tag those for general consumption. So, ability to tag afterwards seems necessary.

This feature involves a script that modify the metadata of the DESCQA runs. The script should check user's identity.

(cc @evevkovacs @katrinheitmann)