Update Mattermost to 5.8 (GitLab 11.9)
Mattermost 5.8 doesn't include many big user-centric features, but it has had some big changes under the hood, notably it includes an image proxy used for loading external images. There was a chance that the image proxy wouldcause problems with images used by the GitLab integration because the image proxy (like the rest of Mattermost) doesn't like to request data from within the local network, but since AllowUntrustedInternalConnections is configured by the Omnibus (link), I don't think that should be any concern. When I was testing, the GitLab icon used for posts made by the integration was showing up correctly, so any other images should work as well.
This release also contains some security fixes that were backported to 5.7.2 and 5.6.5, so I'll be submitting MRs for those as well.
The full changelog for the release is here.