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.
Fixing test for range element in slider.
Updated test for range slider in time slider.

Instead of raw comparison against an HTML string to get style information,
now proper selenium function is used to get value of CSS property.
Fixed acceptance test for start time / end time slider range.

It turned out to be that when you run the tests in
cms/djangoapps/contentstore/features/video.feature
all at once, then test # 11 starts with transcripts already turned on. When you
run just the test # 11, then transcripts are not available (from previous tests).
That's why to make width consistent, we have to make sure that transcripts are
hidden.
CSS width, and left properties can be decimal numbers.
Fix for test.
Removed accidental files.
Updated code as suggested by reviewers.
Updated code as suggested by reviewers.
Rewrote Jasmine tests for slider range to reuse function. More compact and concise.
Chaining jQuery UI slider() methods.
Added entry to CHANGELOG.rst file.
Fixing merge conflicts in tests.EOF

Fixing merge conflicts in tests.
Merge pull request #1397 from edx/valera/add_visual_feedback_start_end_time_video

Add visual indicator to the interface for start/end times