Add an 'all widgets' template
Add an 'all widgets' template

Summary:
To make it easier for testing broader changes, this adds an
editor template which adds an editor for each of the possible widgets.

Test Plan:
 - `make serve`
 - Visit http://localhost:9000/
 - Click "Insert template..." and then "All widgets (for testing)"
 - See that a bunch of widgets are added, and that there are no JS
   errors.

Reviewers: samiskin, charlie

Reviewed By: charlie

Subscribers: jared

Differential Revision: https://phabricator.khanacademy.org/D28313

learningequality/perseus,ariabuckles/perseus,ariabuckles/perseus,ariabuckles/perseus,learningequality/perseus,ariabuckles/perseus