Move the Gitter alerts to the GitLab Pagerduty account

Currently Gitter's alerts flow through its own Pagerduty account. This is a bit of a waste and it would be more efficient to merge it into the GitLab one.

My advice here is to not do it right away. Instead, perform a review for each alert and only move it across when they are fully understood and under control.

The reason why I'm stating this is because the vast majority of the alerts are silent and they would represent a lot of useless noise that would potentially harm visibility on real problems happening on GitLab.com.

Relevant code

Edited by Eric Eastwood