Mattermost security update (3.10.1)

Mattermost is releasing a security update on July 12th. They've already made it available to us so we can schedule a release:

GitLab 9.3.X -- Mattermost 3.10.1:

Download link: https://releases.mattermost.com/3.10.1/mattermost-team-3.10.1-linux-amd64.tar.gz md5 hash: aeb4a7d90265a67f2b68dc208aabf983

GitLab 9.2.X -- Mattermost 3.9.1:

Download link: https://releases.mattermost.com/3.9.1/mattermost-team-3.9.1-linux-amd64.tar.gz md5 hash: f4afb6e1f96a30b668ef145efb64c35e

GitLab 9.1.X -- Mattermost 3.7.6:

Download link: https://releases.mattermost.com/3.7.6/mattermost-team-3.7.6-linux-amd64.tar.gz md5 hash: 3447bfc35c490cd311a9bf671662418b

This will be include with our next security release.