Improve handling of email notifications when MS Teams Connector is enabled

[jasonblais]

When MS Teams Connector is enabled, messages between Mattermost & MS Teams get synchronized.

Currently email notifications are triggered by both MS Teams & Mattermost, which is leading to user confusion.

One proposed approach is as follows:

1. If a direct message is sent in Mattermost, synchronized to MS Teams, email notifications should be triggered based on user preferences in Mattermost

   • Will need to consider a case when the MS Teams user does not exist in Mattermost

2. If a direct message is sent in MS Teams, synchronized to Mattermost, email notifications should be triggered based on user preferences in MS Teams

   • Will need to consider a case when the Mattermost user does not exist in MS Teams

3. If any user who ran the command /msteams-sync disconnected, no email notifications should be triggered by Mattermost

[mkdbns]

Email notifications are handled by their underlying platforms (mattermost / teams). The plugin currently has no known way to control email or desktop notification delivery based on these custom rules.