Make sure that we properly parse and save section release times

Firefox wasn't saving section release times, due to issues with JS Date() parsing.
I've modified the code to make it more explicit around what it should do and how
it should work, which also makes it work better with both Firefox and Chrome.
Cleanup/reformatting
Properly parse and save section release times
Fixed Jasmine tests
Removed unused JS functions
Scoped pad2 function to the one place that it's called
Abstracted functionality to get datetime into separate JS function
Don't ignore null datetimes on subsection settings

Clicking "Sync to <course>" should send an AJAX request with the datetimes
set to null, so that the server resets them.
Created a new lettuce test to catch bug, updated other lettuce tests
Assign isExternal JS function to window object

When JS functions are defined with names, they are local variables, and inaccessible
if defined inside a closure. Django-Pipeline concatenates all of our JS into one
big closure. This function explicitly assings the function to a property of the
`window` object, so that it is accessible to other JS functions.
Merge pull request #604 from edx/hotfix/2013-08-07

Hotfix/2013 08 07