User flag to set notifications to opt-in instead of opt-out

**Is your suggestion related to a problem? Please describe.**
As I add more rooms to Matrix, I'm realizing that I want the default setting for public rooms to be "mute notifications", because I also use Matrix for work, and so I have it set so that only urgent work channels provide a notification.

**Describe the solution you'd like**
A setting for notifications inside the user settings, with three switches

- mute notifications upon entering:
- a public room
- a private room
- a chat

Which should probably contain an explanation along the lines of "notification settings can still be changed on a per-room basis"

**Describe alternatives you've considered**
This is it, the only alternative I could think of would be to just default all users to no notifications upon join, and I don't think that would be appropriate, since that would make notifications a discoverable feature, meaning some users would never discover it, and would simply think that notifications are broken or not provided.

**Additional context**
Add any other context or screenshots about the feature request here.