Support adding space-restricted joins on rooms not members of those spaces

[turt2live]

Our workflow for new rooms internally appears to be:

1. Create room
2. Try and restrict it to joins for members of Element Corp.
3. Get weird "No results" message
4. Ask around internally for it to be added
5. Coordinate an invite into the room to add it to Element Corp. (because you need to be a member of the room to add it to a space, rightfully)
6. Add it to the space
7. Make it restricted to members of that space

With this PR, the new flow would be:

1. Create room
2. Make it joinable to Element Corp
3. Ask around for it to be added to the space
4. It is added to the space

There is a small risk that steps 3 & 4 get missed, but would kinda like to see what happens in practice. This should be trivial to revert if we find it encourages bad behaviour.

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Code reviewer: I have not added tests to this yet. The intent is to get design/product signoff first in case the UI changes dramatically. This is really meant as a quick-win PR, so frontloading tests feels wrong :)

Blockers:

• [ ] Tests (needs design/product signoff first)

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Fixes https://github.com/vector-im/element-web/issues/19213

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Here's what your changelog entry will look like:

✨ Features

• Support adding space-restricted joins on rooms not members of those spaces (#9017). Fixes vector-im/element-web#19213.