Fix custom TinyMCE init selectors. Don't cache a page's selectors in a class variable.
Fix error when clicking 'add new content' due to JSON.parse of ''

When clicking 'add new content' in admin for something with available_contents, you'll the following error:
JSON::ParserError A JSON text must at least contain two octets!

This is because params[:options] is an empty string and is then passed to JSON.parse().
Merge pull request #402 from wuservices/2.7-stable

Fix custom TinyMCE init selectors