Anton/video player grading

**Ticket:** [BLD-1020](https://edx-wiki.atlassian.net/browse/BLD-1020)

**Description:**
This PR adds video player grading functionality.
To enable the functionality go to `advanced setting` find `video_grades` field and set it as `true`, then you can find new fields in the video player editor (`Advanced` tab):
A studio editor with a text box and two dropdowns. 
First dropdown defines whether students are graded for watching the video. Watching any portion of the video or downloading the video counts if neither of the other two options are selected. 
Text box (with validation for a percentage) allows author to select the percentage of video watched.
Dropdown allows author to select whether the student must reach the end. Downloading the video counts as reaching the end.

@jmclaus  - javascript part 
@srpearce - documentation, labels
@marcotuts - UI, UX
@rocha - logging
@muhammad-ammar - bok-choy tests

@sarina please review python part.

**Sandbox**:
http://studio.polesye.m.sandbox.edx.org/