per-space notification options

**Is your suggestion related to a problem? Please describe.**
When I want to stop notifications (or change setting) on all rooms in the same Community, I have to do it one by one (same for DMs)

**Describe the solution you'd like**
It would be nice to have an option to set notification options on a Community level

Start by reviewing how Element clients currently expose notification settings for Communities, rooms, and DMs, then read the seven existing comments for constraints or decisions. Define the intended per-Community behavior and its interaction with individual room and DM settings; done means users can change one setting for all relevant rooms instead of editing them one by one.

Written by the indexing model from the issue text.