First iteration of Checklist extension

Milestone: Checklist on own page, can add/delete items and check items off
           Changes persist in database

Includes:

Extension configuration files
Django models
Backbone models/views

Remaining work:

Currently hardcoded to request ChecklistInstance with id=1. Need to figure out how to create or retrieve a ChecklistInstance associated with the current Review(Draft)? in progress.

Currently need to hardcode first Checklist / ChecklistInstance in, with the JSON = [] (empty list)

Went to home page, extension page: checklist does not appear
Go to a review request: checklist appears. Should be visible on all review views.
Add items by filling in textfield and hitting enter
Update items by double-clicking their text, editing the text, and hitting enter
Delete item same way as updating, but set text to be empty
Check and uncheck items
Refresh or leave the review, then come back and confirm changes persist.
Drag the dialog around. Scroll the page. Dialog should stay in place.
Attached a screenshot and opened it. Checklist is still visible.