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
Converter validation: don't call removed_cached_runtime_json  [#67314208]

https://www.pivotaltracker.com/story/show/67314208
Finish runtime caching, begin runtime json errors

completes [67856314]
starts [#67314208]

https://www.pivotaltracker.com/story/show/67314208
https://www.pivotaltracker.com/story/show/67856314

Activities can by_pass the runtime json conversion w/ prevent_conversion

This is handy when being copied, or when running certain tests.

TODO: Maybe also specify a way to prevent running conversions using ENV
for testing scenarios?

whitespace cleanup.

This method was moved to sg_permissions.

Test for error detection in converter.

Invalidate chache when name or author_name changes.