Video: Fix speed persistence per video.

In this PR was fixed bug with speed persistence of video player.
Appropriate acceptance test was fixed to cover this case.

@auraz please review.

Steps to reproduce:
```lettuce
Given I am registered for the course "test_course"
    And it has a video "A" in "Youtube" mode in position "1" of sequential
    And a video "B" in "Youtube" mode in position "2" of sequential
    And a video "C" in "Youtube" mode in position "3" of sequential
    And I open the section with videos
    And I select the "2.0" speed on video "A"
    And I select the "0.50" speed on video "B"
    When I open video "C"
    Then video "C" should start playing at speed "0.50"
    When I open video "A"
    Then video "A" should start playing at speed "2.0"
    And I reload the page
    When I open video "A"
    Then video "A" should start playing at speed "2.0"
    And I select the "1.0" speed on video "A"
    When I open video "B"
    Then video "B" should start playing at speed "0.50"
    When I open video "C"
    Then video "C" should start playing at speed "1.0"
```