Updated Mattermost to 4.8.2 (10.7)

Harrison Healeyrequested to merge
mattermost/omnibus-gitlab:mattermost-4.8.2 into 10-7-stable

Mattermost 4.8.2 contains a fix for a minor security issue.

If this should be added to the GitLab changelog, please let me know.

@jasonblais @marin

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