Mattermost v5.16.3 for GitLab v12.5

@twk3 I'm opening an issue to give your team a heads up that we have prepared a dot release 5.16.3 to fix the following bug on our latest release:

This issue was also backported to Mattermost 5.15.3 (GitLab 12.4).

We will prepare a merge request to GitLab omnibus in coming days.

Let me know if you have any questions.