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:

• (Accessibility) Fixed an issue where keyboard navigation within the right-hand side did not navigate in expected order. https://mattermost.atlassian.net/browse/MM-19901

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.