Switching Mattermost Gitlab ID & secret around to be consistent

Simple change but it cost be a few minutes of head scratching this morning!

When creating a new application link in Gitlab you are presented with the ID, followed by the secret. All other places in gitlab.rb follow this pattern of ID first except for the Mattermost section. If you aren't paying enough attention (as I wasn't this morning) and are blindly copy/pasting, you could easily put the ID where the secret should be and vice versa.

This MR simply switches the config around to remain consistent.