[Task] Create a Space screen.

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This is the nested screen shown when tapping a space in the Spaces tab.

Please see and discuss the details in the meta issue.

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All done other than the ability to show the name of the room in the header when there's a restricted join rule.

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The allow rules of the restricted join rule aren't given to us by Synapse, so #4544 manually passes in the parent space (by navigation) for now as a workaround.