Clear cached versions of when activity parts are added or destroyed.

[#67856314]

https://www.pivotaltracker.com/story/show/67856314
whitespace.
Move more caching behavior out of controller to the caching module.

[#67856314]

https://www.pivotaltracker.com/story/show/67856314
Small simplification: Reuse the activity we looked up elsewhere.
Write to FS from the caching module.

[#67856314]

https://www.pivotaltracker.com/story/show/67856314
Cache entries after successful authoring.
Rename caching module to sg_activity_caching.rb rename some methods too.

json_cache_path ➙ cache_path(filename)

Should finish [#67856314]
https://www.pivotaltracker.com/story/show/67856314
Create the path to the cache file if it doesn't exist yet.
Hide some fields from forms. EG: Activity errors.
Clear old errors. Don't let runtime errors prevent active record saves.
Visual feedback when generator errors crop up.
Capture error message from converter, present to author on failure.

[#67314208]

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