Implement UI for cross-signing/SAS with other users

- Try to use same sub-views used for device verification with the parent being the right panel.
- Open the dm with the user using MemberDetailsViewModel.openDirectMessage.
- Add support for `reference` relations in RelationWriter so that the event entry for the request has an array of related reference events. You can use this in the tile get an array of related events. Don't do this if possible and instead check if the event was loaded from storage and if so render the tile as completed. Only show button on events that come fresh through sync. Will need to implement a flag that tells us whether the event came from storage or sync.