On certain modal dimensions, the black background of the video
preview could peek through, making it look like the video has a
black line. The change is to remove the black background from
showing by default and having it only display when the video is
muted. Also, the video preview dimension stylings have been
changed to facilitiate smoother size adjusting with modal size
changes.

Instead of using AtlasKit Single-Select, use Dropdown. Dropdown
differs in that an icon can be specified for the trigger element,
whereas Single-Select currently supports icons for all elements,
and Dropdown can show all options incuding the already-selected
option.

This change does introduce the issue of the trigger element not
taking up 100% width of the parent. Supporting such would involve
overriding AtlasKit CSS. The compromise made here was to do a
generic override of max-width so the trigger elements at least
stay within the parent and aligning the trigger elements to the
right.

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.