Fix some issues that were breaking javascript in firefox.

This fixes a regression introduced when marked.js was removed. Apparently
testing global names to see if they're defined doesn't work so well in firefox.
This broke pretty much all javascript everywhere. Fortunately, we don't care
about setting any marked.js options anymore, so this can just go away.

This also fixes a missing semicolon.

Testing done:
I can edit things on review requests again.

Reviewed at https://reviews.reviewboard.org/r/6440/
Fix a regression in the review request API.

The recent change to the close description fields to handle markdown caused a
regression in the case where the close_description field was null. In this
case, trying to normalize the text would fail, and the API endpoint would
return a 500 with a traceback.

This change makes it so that if a text field is null, we just pass that
through.

Testing done:
- Verified that I could once again update un-published review request drafts.
- Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/6441/
Fix an occasional problem with the new CodeMirror optimizations.

We were throttling how often the 'change' event handler for CodeMirror
was getting invoked. However, this meant that there was a window of time
where CodeMirror could be closed/destroyed before the 'change' event
handler was called.

We now check for this and bail if CodeMirror is no longer with us.

Reviewed at https://reviews.reviewboard.org/r/6439/