Fix whitespace problems in xblock string field editor.

Whitespace is now trimmed from input. If after trimming input is empty
(pure whitespace or the empty string), the field is not updated.
Adds warning when editing a container visible to students

styling for editing a live unit
Merge pull request #4327 from edx/benmcmorran/visibility-warning

Adds warning when editing a container visible to students
Merge pull request #4338 from edx/dan-f/display-name-validation

Dan f/display name validation
Displays the date, time, and username of the most recent save and publish