Default settings for gitlab-mattermost

In !1128 (merged) we've set a new default value for mattermost, and we'd like to set some more new values. Given this might override behaviour admins now expect, @markpundsack pointed out this might not be the best idea.

However, for new installs we'd still like to set these values. So my idea was to:

1. Revert !1128 (merged)
2. In the gitlab.rb template, uncomment the values we'd like to set a default for, with the set value we'd like
3. At a 9.0 release make the overridden defaults really a default

This way we'll only impact new installs, until 9.0.

/cc @ayufan