[IMPROVEMENT] Unified header UX (#2234)

* Change drawer icon

* Removed iOS variation

* Patch to react-navigation-header-buttons... easier to patch then to overwrite its behaviour :(

* Correctly position title

* Header subtitle

* Layout

* Alignment

* RoomView header

* Renamed RoomHeaderLeft to LeftButtons

* RoomView back button

* Search icon on RoomView

* Refactor

* Fix header on tablet

* Fix search messages close button on tablet

* Search key command

* Network status on RoomView header subtitle

* Update tests

* Scale content

* SearchBox cancel color