Update Mattermost to 5.18.1
This is based off !3846 (merged) and is intended to be merged after it has gone in.
Mattermost 5.18 is a feature release containing new features such as the ability to mark channels as unread. The full changelog for it is here. Mattermost 5.18.1 also contains security fixes that have been shipped with Mattermost 5.16.5 (!3845 (merged)) and 5.17.3 (!3846 (merged))..
Edited by GitLab Release Tools Bot