So after many hours of configuring Mattermost, I have as far as I am aware only one issue left.
I am using GitLab Mattermost on CentOS 6, and I am using Apache/2.2.15.
I do not know of any way to proxy WebSockets on Apache unless you have Apache 2.4 or higher, or compile the entirety of Apache again with a couple of patches.
If there is no way to proxy WebSockets through port 443 without having a large chance of breaking things, can I configure Mattermost not to annoy users by showing the following popup?: "Please check connection, Mattermost unreachable. If issue persists, ask administrator to check WebSocket port."
I mean, that popup comes back after a second when clicking it away.