Allow Space or Room admins to set the default notification level

(I'm aware this is a spec and backend change, I just want to note this down as a feature I'd like as a user of Element)

A-la discord;


(That link leads to [this](https://www.youtube.com/watch?v=zGl796352RI), btw)

#### Is your suggestion related to a problem? Please describe.
As an admin of a space or a room, I'd probably like to be most friendly to incoming users, to allow them to have the best time.

#### Describe the solution you'd like.
For this, I'd probably want to fine-tune a Default Notification Level, to draw attention to all messages to a certain room, leave messages of some room be able to be mention-only, or (maybe) set some rooms to not generate notifications at all.
(see above screenshots from discord)

This would only apply to when users join a room for the first time (or any time, if it's not possible to assert a user hasn't been in the room before), then the server (or client) would default the notification level to one an admin sets.

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This is related to https://github.com/vector-im/element-web/issues/17361, but unlike that issue, this would call to make admins of spaces and rooms be able to set that notification level.

No files, tests, or entry points are named. Start by reviewing the related issue #17361 and clarifying the required server/client ownership and first-join behavior; done means an agreed specification and implementation path for admin-configurable default notification levels on rooms and Spaces.

Written by the indexing model from the issue text.