Create app

Create django application providing annotator-storage functionality.

https://openedx.atlassian.net/browse/TNL-780
two things about the "ignore" tests:
* is it an explicit design decision to ignore unexpected fields in request payloads, rather than reject them (HTTP 400)?
* if we are going to test for specific unexpected / ignored fields, i suggest some sort of factoring so that it doesn't require 10 new lines of boilerplate for each one.