Anton/store video progress

**persist student progress in video**

The following test cases are achieved with this PR:

    Student logins to edx, plays video, selects position,
    closes browser, opens video, position is restored

    Given a video with a start time of :30 and an end time of 1:00
    And a student who navigated away from the video was at :45
        (in between start time and end time)
    When the student returns to the same video
    Then the video should resume playing from :45.

    Given a video with a start time of :30 and an end time of 1:00
    And a student who navigated away from the video when the slider was at :15
    When the student returns to the same video
    Then the video should resume playing from :30

    Given a video with a start time of :30 and an end time of 1:00
    And a student who navigated away from the video after moving the video to 1:15,
    When the student returns to the same video
    Then the video should resume playing from :30

**JIRA issue**

[BLD-385](https://edx-wiki.atlassian.net/browse/BLD-385)

**Reviewers**

- @singingwolfboy 
- @polesye