move mattermost to our own server

[edgaleano]

1. move mattermost
2. update DNS; point chat.vokomokum.nl to mattermost
3. write maintenance documentation

Once we've got this working contact people from the communications working group so that they can start to test and use mattermost. Currently they're trying to setup Discord, but Mattermost is better :)

[nualoy]

Instead of moving the test instance of Mattermost, I installed a new one via Omnibus, which also packages and configures the PostgreSQL database and Nginx as the application proxy with auto-renewed SSL certificates. This will make it way easier to maintain. The documentation is available online: https://docs.mattermost.com/install/installing-mattermost-omnibus.html

https://chat.vokomokum.nl

One small pending thing is adding the SMTP configuration so it can send e-mails. After that, we can open it up to the whole group.