Course Updates date validation

TNL-4115. Previously, course updates (which are intended to be posted with
dates, for sorting in the LMS) could be authored in studio with a valid
date, nothing, or a random string as the "date" for the update. As there
is no validation for this in studio, everything succeeded with no warning.

However, the LMS has problems parsing some of these values, and barfs when
loaded by learners.

The fix does two big things:
- gracefully handles invalid dates in LMS. These updates are now treated as
having a date of today, for sorting purposes.
- turns on validation in studio. Now, it is not only impossible to enter
invalid dates in studio, but notifications will draw the course author's
eye if any invalid updates were previously saved.

Test additions for this commit:

Adds:
- unit test for LMS parsing
- Jasmine test to confirm invalid dates cannot be set by the user
    -also adds event to setAndValidate instead of using a global object
- fix for lettuce test
    -It is no longer valid to enter the string "January 1, 2013" as this test
    had been doing. Keyed-in entries must use MM/DD/YY format.

cms/djangoapps/contentstore/features/course-updates.feature

@@ -34,7 +34,7 @@ Feature: CMS.Course updates
         Given I have opened a new course in Studio
         And I go to the course updates page
         And I add a new update with the text "Hello"
-        When I edit the date to "June 1, 2013"
+        When I edit the date to "06/01/13"
         Then I should see the date "June 1, 2013"
         And I see a "saving" notification