Remove marked.js and google-code-prettify.

Now that we no longer render Markdown client-side, we have no need for
either marked.js or google-code-prettify. This change removes them.
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/
Merge branch 'release-2.0.x'