Add NSUserActivity to rooms to appear in spotlight search

[kloenk]

This also allows shortcuts to open the app directly opening a room.

Pull Request Checklist

• [x] I read the contributing guide
• [ ] UI change has been tested on both light and dark themes, in portrait and landscape orientations and on iPhone and iPad simulators
• [x] Pull request is based on the develop branch
• [x] Pull request contains a changelog file in ./changelog.d
• [ ] Pull request includes screenshots or videos of UI changes
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[pixlwave]

For this initial implementation, would you mind removing the CSSearchableItemAttributeSet and CSSearchableItem parts and relying on the userActivity.isEligibleForSearch = true for spotlight support? I believe the only visible change this would have is the lack of a subtitle - which may be a positive thing as it could be confusing in the Shortcuts app.

It would be interesting to have spotlight functionality in its own service eventually, so that updates to the index can happen automatically. Then the UserActivityService could request the appropriate attributes to attach to the activity from that service.

[kloenk]

Let's anchor 3086234a479bfcd84b6106eb1a5831f6f4bbe337 here, so it's easier to find how to add it back later on

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