Update Mattermost to 5.2.1
The full changelog for Mattermost 5.2.1 and 5.2.0 is available here.
The most notable change in this release is a substantial overhaul of the plugin system, although it's still in beta at this time. Apart from fixing it so that GitLab can once again properly configure the location of the plugins (once !2649 (merged) is merged), most of the changes are around the API used by the plugins, so the main change for users is that they'll need to update their plugins for 5.2 if they are somehow using them.