WIP: Updating to Mattermost 3.1

@marin just spoke with @stanhu about opening a Merge Request for updating Mattermost 3.1 to start a discussion.

@stanhu mentioned you were highly backlogged and that we should try to find ways to help where we can, potentially reduce scope as well.

I'm not super deep on how omnibus works, I think the changes would be:

1. Add settings to gitlab.rb to run an uninterrupted upgrade via ./bin/platform -upgrade_db_30 -confirm_backup="YES" -team_name="yoururlteamname"

2. If those fields aren't filled out, then don't restart the Mattermost server instead output something like the following:

Additional step are needed before the upgrade to Mattermost 3.1 is complete. 

Mattermost 3.1 offers multi-team accounts. When upgrading to this version, users with duplicate accounts on different teams will have their duplicate accounts renamed. 

To complete the upgrade: 

1. Please back up your Mattermost database and in `gitlab.rb` update `confirm_backup` to `YES`.
2. In `gitlab.rb` update `team_name` with the name of the primary team you use (user accounts in this team will be preserved, and their duplicate accounts on other teams will be renamed)
3. Run `gitlab reconfigure` to apply the settings.

Mattermost 3.1 will then upgrade your database. Any users with duplicate accounts will receive an email explaining how multi-team accounts work, and how their duplicate accounts were renamed. 

For more information, please see: http://www.mattermost.org/upgrading-to-gitlab-mattermost-3-1/

For troubleshooting, please see: http://www.mattermost.org/troubleshoot/

I think this would cover the minimum needed to get Mattermost 3.1 working.

Thoughts?