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.

* ref(popover): allow for popover content from the right

Popovers contents can display to the left of the trigger
and above the trigger. Add the ability to display to the
right of the trigger my adding mouseover padding. This
may be needed for tile view, depending on where the triggers
are located.

* squash: abstract common css proprties into placeholder class

Prepare for React 16.