Item editing permalinks

Summary:
Adds a permalink button that gives you a link to that item's content
This means you can stop having to save things in test.html to have that
appear when reloading the page.

Test Plan:
 * add a random widget
 * press permalink
 * copy-paste the URL into another page
 * reload the page

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6584