Moving unregistration of change listener into asset_index.html.

Fixes broken integration test on master (the integration test was related to setting the release date of a section-- when the modal was closed, it was going through the code in base.js that tried to unbind the change listener on something unrelated and non-existent).

I verified:

1) base.js does not call any of the other methods I changed from global to local variables in asset_index.html
2) Course import works properly (there are a couple of places where '.file-input' appears in base.js, related to course import). Course import did not use the startUpload callback.

@jkarni Can you please review? I will kick off the tests manually in Jenkins.

@wedaly This is the fix for the broken integration test. I had changed "startUpload" so it was no longer globally defined, but one place in base.js was attempting to call it.