Allow email notifications to be disabled for all users of a group

Note from @ebrinkman on 2019-06-20: We should adjust this to only be at the group level unless the community contribution is nearly complete with both group and project level toggles.

Allows Owner only user permission level of groups to disable email notifications on a per-group, regardless of the users settings.

Design

• Need 'Owner' permissions to update this on group level
• All areas where a user can update notifications should be disabled with a tooltip that says "Notifications have been disabled in group settings"
• Preference should be added to settings
  • group > General Settings > Permissions, LFS, 2FA