Atlaskit at times will have localized styling for font-size and
sometimes will not. The button component will inherit its
font-size whereas selectors have localized font-size of 14px. For
consistency, the cancel/submit buttons on the atlaskit modals
will also have 14px. The atlaskit story book examples also use
buttons with 14px font-size.

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.

#1411