a11y - Videos need to alert screenreaders when the video is over.

BLD-488

<b>Description</b>
When a blind user uses a screenreader to watch videos, he is not aware of when the video ends. When the last transcript line is read by the screen reader, and the video ends, nothing happens to inform the user that the video ended.

<b>Solution</b>
When the video ends (either on end time, or at the end), the `title` attribute of the time slider knob is set to `video ended`. The knob is brought into focus for the screenreader to read out the title attribute. Because the know is the first element to get focus when the user is tabbing through video controls, upon further tabbing, the next element in line after the know is the play button. When the video starts playing again, the `title` attribute is reset back to `video position`.

<b>Reviewers</b>
- @wedaly