* Move ComposerToolbar files to withing the RoomScreen folder (as it's not a screen on its own)
* Switch the plain composer to NSAttributedStrings
* Enable suggestions on the plain composer
* Introduce a new `MatrixUserDisplayName` attributed string attributed and use it for mention building
* Implement mention pill rendering and sending on the plain message composer
* Fix plain composer snapshot tests
* Fix broken formatting options layout
* Add clarifying comment.
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