fix(mattermost): Missing configuration key for same-server setup

The documentation states that "GitLab Mattermost is disabled by default," following that only the mattermost_external_url should be set to enable the service, which seems wrong, as you need to set mattermost['enable'] as well. The gitlab.rb has it disabled per default and not setting it will just disable the service at startup.

PS: Not sure about the gitlab_rails['enable'] either in the "own-server" section, searched the whole project for that but it does not give a single hit except for the documentation page. Maybe its deprecated or was changed fundamentally?