* Skips setting undefined device id to sink in audio preview.

* Fallbacks to use labels for user selected devices.

* Fixes comment.

The Audio.js setRef callback does not behave like react ref callback
in that the former will not have fired before componentDidMount
but the later will have. So for audio output preview, trying to set
sink id on mount will no-op because it does not have a ref yet to
Audio.js, possibly leading to audio output previews playing on
the default speaker device. This generally has not been a user
visible problem due to coincidence; other re-renders necessary
by the parent of audio output preview will have triggered
componentDidUpdates on the audio out preview, which would then
set the sink id on the Audio.js ref it should have received
by then.

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.

The Device Selection modal consists of:
- DeviceSelection, an overly smart component responsible for
  triggering stream creation and cleanup.
- DeviceSelector for selector elements.
- VideoInputPreview for displaying a video preview.
- AudioInputPreview for displaying a volume meter.
- AudioOutputPreview for a test sound output link.

Store changes include is primarily storing the list of
available devices in redux. Other app state has been left
alone for future refactoring.