Feature: Make ansibot update existing comment when updating cc-list

[dagwieers]

So currently if we add a new member to a team in BOTMETA.yml, all existing members will be notified for every issue that relates to the team. I recently added a new "notified" member to the Windows team, and the 150 issues and 50 PRs all got updated with a cc: to this new member. That's 200 mails (and notifications) for every existing member.

What could help is if ansibot would update the existing cc-comment with additional members to cc. This would automatically subscribe this new member to these issues, without also causing existing members to get all these notifications.

(If there was an API-call to update the subscriber-list, that would be useful as well, although less visible to the team).

This relates to #1081 -- Make ansibot less chatty.

cc @gundalow