Prepare for React 16.

- Create new React Components for RemoteVideoMenu and its
  buttons
- Remove existing menu creation from RemoteVideo
- Refactor RemoteVideo so all function binding happens once in
  the constructor, removing the need to rebind when updating
  the RemoteVideoMenu
- Allow popover to append and remove React Components from itself
- Refactor popover so post-popover creation calls are broken out and
  popover removal behavior is all done in one function.