Sliding Sync: improve sort order, show subspace rooms, better tombstoned room handling

[kegsay]

Relies on matrix-org/matrix-js-sdk#2801

This does 3 things:

• Change the sorting order for lists from "by_highlight_count", "by_notification_count" to "by_notification_level" which more accurately represents the sort order used in non-SS EW (group rooms by highlight/notif/none then sort by recency).
• Remove is_tombstoned which is now deprecated as SS will automatically exclude old rooms. Replace with include_old_rooms to opt-in to old rooms for certain lists (currently all of them but I don't know which are really required by the app.
• When the active space changes, include subspace room IDs in the room list filter to ensure we see rooms from subspaces in the room list.

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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Here's what your changelog entry will look like:

✨ Features

• Sliding Sync: improve sort order, show subspace rooms, better tombstoned room handling (#9484).

[SimonBrandner]

LGTM