* pubsub doesn't appear to have any alerting, let's alert when things
get backed up in the pubsub arena
  * Since this is not an outage, only send it through slack
* Specific to gitaly, let's create an alert if the amount of data he's
sending is super low
  * Since this is not an outage, only send it through slack

* While I'm in here, moves a rando gitaly alert to a central place for
all gitaly alert configurations

Signed-off-by: John T Skarbek <jtslear@gmail.com>