nmbug.js: Fill in the POST /mid/<message_id> handling

nmbug.js: Fill in the POST /mid/<message_id> handling

This is currently only (un)setting one tag per request.  If we want, a
later optimization could queue changes and push them all at once (e.g.
after a user pushes a submit button).  It would also be nice to update
the UI based on the list of returned tags, so we hear about changes
made by parallel parties.