This PR adds a release for Mattermost. The code is fetched from the GitHub repo and built.
Current state: Mattermost works. Default configuration allows user registration. Push notifications work if enabled in the settings.
To be done:
- request a domain/frontend and write it as the root URL to the configuration file
Rationale: I was working on this for the purpose of finding an alternative to Jabber, but it seems we will be moving forward with Delta Chat instead. The SR was almost finished anyways, it's always good to have.