Mattermost upgrades to 3.7.3 and 3.6.5

Mattermost is being upgraded to 3.7.3 for GitLab 9.0.4 and 3.6.5 for GitLab versions 8.17 and 8.16. These versions contain important security fixes.

MR:

!1426 (merged)

!1450 (merged)