Mattermost 5.14: Upcoming breaking changes

@twk3 Below is a note of upcoming breaking changes that would affect the Mattermost integration with GitLab Omnibus.

Mattermost v5.14 (GitLab 12.3 - September 22)

• Enable Plugin Health Check is removed from config.json. Instead, all plugins are now automatically monitored and restarted if a plugin has crashed or is deemed unhealthy. If a plugin crashes three times within an hour, the server automatically deactivates the plugin.
• Webhooks are now only displayed if the user is the creator of the webhook or a system administrator.
• With the update from Google+ to Google People, system admins need to ensure the GoogleSettings.Scope config.json setting is set to profile email and UserAPIEndpoint setting should be set to https://people.googleapis.com/v1/people/me?personFields=names,emailAddresses,nicknames,metadata per updated documentation.