Create collection endpoint

Endpoint in backdrop to create a new capped or uncapped collection.
- Added tests in "English" using python-behave
- Created new rules for python-behave to implement new tests
- Add two new methods to the Database class
- Implement the actual endpoint
- Add new special token ``CREATE_COLLECTION_ENDPOINT_TOKEN`` which will be shared with Stagecraft

See story https://www.pivotaltracker.com/story/show/63958958 [#63958958]