Ability to mute rooms per device rather than account wide

**Suggestion**

Sometimes it would be nice to have a room on loud but muted on a certain device. Currently this is not possible.

The specific usecase for me is that since there is no nice multi account support in element clients I have my work rooms on my Main account. However I dont want these chats to give me push notifications on my phone as usually it means I am out of Office anyway. It would be nice to keep them active at the PC but not on my phone therefor.

No files, tests, or entry points are named. First clarify how per-device room muting should be represented in the Matrix specification and what protocol behavior is required; done means the specification defines the behavior clearly enough for clients to implement it.

Written by the indexing model from the issue text.