Initial commit for feature BLD-391: Add arrows to the interface for start/end times.

If end time is specified (which means that there is also a start time), we
add an extra DOM element to the slider that will show the are in time between
start time and end time. Unfortunately, this visual aid will become available
ony after start of playback. This is because until then we don't know the length
of the video.
Fine grain control of when to show the range.

Also moved all static styling to SCSS.
Fixing Jasmine tests. Adding tests for start end time checking.
Initial sketch of acceptance test for start end time range in slider.