GitLab SSO login throught NAT gateway

[amy.blais]

Hi @Junn-Sorran,

Wondering if any of these previous threads might help:

• https://github.com/mattermost/mattermost-server/issues/1899
• [Solved] "The redirect URI included is not valid." after mmost URL change
• "The redirect URI is not valid" during OAuth on on new Gitlab v8.2.2 omnibus installation
• Oauth not valid behind reverse proxy