GitLab Mattermost: Mapping `mattermost_external_url` to `ServiceSettings.SiteURL`

In Mattermost version 3.6.7, 3.7.5 and 3.8.x and later, the Site URL setting is required for email notifications and email invitations. If the Site URL is not set, the System Admin sees a header bar notification until the setting is specified in the gitlab.rb.

To ensure a smooth upgrade for those instances of GitLab Mattermost that do not have Site URL specified, we recommend a minor patch to set the value of mattermost_external_url in gitlab.rb as the value of our ServiceSettings.SiteURL setting when creating the config.json for Mattermost.