From Community Issue: https://github.com/mattermost/platform/issues/1761
a) In Account Settings > Notifications add an option to enable mobile push notifications for certain activity. Use radio buttons for the three available selections:
Title: Mobile Push Notifications
[ x ] For mentions and direct messages
[ ] For all activity
[ ] Off
Help Text: "Notification alerts are pushed to your mobile device when there is activity in Mattermost."