Too many individual users occasionally don't update their mirrors,
causing us to be permanently alerting; we have sufficient user
notification at this point (plus Waseem keeping an eye on /activity)
that we don't need to alert on individual users.
We do, however, still care if something happens (say, Linerva going
down) that causes many users' mirrors to go down.
(imported from commit 392952c95739e183d4a711120e3a963671cec289)
It will alert when our users' mirrors don't appear to be running, as
assessed by having recently made a get_message API request.
(imported from commit 4b8c5f51b007568a90a92f7b095c51f3566d5117)
It checks the output of api/bots/check-mirroring and alerts if we
aren't able to send and receive mirrored Zephyrs.
(imported from commit 6c9abc380fca955d00462f829fa7dcadfef24221)
