Update Mattermost to 5.27.0 and identify Omnibus installs in MM
Mattermost 5.27.0 is a quality release, meaning it has no major features and only contains minor changes such as bug fixes. The full changelog for it is here. Note that this may be the last of these quality releases since we're changing our release process in the next month or two, although I don't think that will have an effect on the GitLab side of things outside of adding features more frequently.
One extra addition in this release is that we've started adding better analytics to Mattermost, and one of the things that we're wanting to identify is differences between different types of deployments, such as whether or not Mattermost is installed as part of GitLab Omnibus. This is mostly handled by Mattermost itself, but I had to add an additional environment variable to actually tell Mattermost that it's running under GitLab Omnibus. For anyone who wants to disable these, they're controlled by the same setting as previous Mattermost analytics were, so there won't be any change there. More information is available on our ticket MM-28118.
This release contains some minor security fixes, none of which are significant enough to be backported to previous Mattermost releases.