Video: Add highlighting of start/end time field when clicking into it.

[BLD-581](https://edx-wiki.atlassian.net/browse/BLD-581)

**Description**
When clicking into the start/end time field, the entire 00:00:00 string should be highlighted so that when the user starts typing, she is (by default) typing in an empty text field.

Studio -> Video -> Editor -> Advanced tab

@jmclaus