Nominate a leading device for Notifications

[daniellekirkwood]

It's frustrating to receive a notification on every device for Element when I'm already at my computer looking at Element... If we nominate a 'lead' device that mute other deivces when you're on it that might be a way to get around this (similar to slack)

Interesting screenshots/ideas below:

[t3chguy]

Duplicate of #360