Update Mattermost to 5.16.2
Mattermost 5.16 contains some new features such as Guest Accounts, the Plugin Marketplace, and improved user management tools for System Admins. It also contains some security fixes that have also been included in Mattermost 5.15.2 and 5.14.5. The full changelog is available here.
Note that this MR is based upon !3735 (merged) since that's also being submitted against master