Autosave all the protips - clientside of course. [WIP 146]

Work for WIP #146 - https://assembly.com/coderwall/wips/146

Current behavior:
* Once a user starts typing, we save the Protip text area content every 3 seconds.  Delay can be configured if we want.
* If content has been saved, and a user navigates away from the page (or has a browser crash, etc) - when they come back to the new Protip form, their autosaved content will be populated in the text area from localStorage.
* When a user submits the form successfully, the autosaved text area content is cleared.  The next request to the new Protip form will not populate the text area with any localStorage content.
* When a user submits the form unsuccessfully (validation errors) the form will maintain it's field content from the form submission and store the prefilled text area content into localStorage.
add autosaved to assets.rb