For the most part the changes are taking the "static propTypes" declaration off
of components and declaring them as Flow types. Sometimes to support flow some
method signatures had to be added. There are some exceptions in which more had
to be done to tame the beast:
- AbstractVideoTrack: put in additional truthy checks for videoTrack.
- Video: add truthy checks for the _videoElement ref.
- shouldRenderVideoTrack function: Some component could pass null for the
  videoTrack argument and Flow wanted that called out explicitly.
- DisplayName: Add a truthy check for the input ref before acting on it.
- NumbersList: Move array checks inline for Flow to comprehend array methods
  could be called. Add type checks in the Object.entries loop as the value is
  assumed to be a mixed type by Flow.
- AbstractToolbarButton: add additional truthy check for passed in type.

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.