clean up tinymce_helper.rb
Cache responses for author_preview.html student_preview.html [#67856314]

https://www.pivotaltracker.com/story/show/67856314
Invalidate the activity cache when activities are modified in any way.

[#67856314]

https://www.pivotaltracker.com/story/show/67856314
ignore cache directory.
WIP: The converter knows when it has failed. Part of [#67314208]

https://www.pivotaltracker.com/story/show/67314208
Check that the converter runs cleanly.  [#67314208]

https://www.pivotaltracker.com/story/show/67314208