Video: Update behavior of start/end time fields.

[BLD-506](https://edx-wiki.atlassian.net/browse/BLD-506).

It should be possible to enter the start/end time as a string HH:MM:SS, and only have the field adjust after the user clicks away. If the user enters something like "1:35", we would interpret that as "00:01:35" after the user clicks away. However, while the user is entering text into the field, it should show the keys that the user was pressing. If the user enters something completely invalid, reset to 00:00:00.

@jmclaus, @auraz please review.