Don't reapply labels that already exist
Created on August 25, 2017. Copied from redmine (https://bugs.ligo.org/redmine/issues/5714)
Labels that already exist on an event should not be able to be applied. Annoying when ADVOK is applied multiple times and you get multiple phone/text alerts.
Points to consider:
- Should an XMPP alert be sent out if, for example, the advocate signoff label was already ADVOK, but the comment is just changed? Should it still be an "alert for label" or just an update?
- We definitely shouldn't send out any alerts for "normal" labels like INJ, DQV, etc. if they're reapplied.