Chant create: optimize suggested chants feature

[dchiller]

This PR eliminates all calls to the json-cid endpoint for the "suggested chants" feature on the Chant Create view. All data required for the suggested chants is now supplied directly in the response to the json-nextchants endpoint.

Given that we no longer need to make these extra calls, this PR also returns the number of suggested chants presented to at most 5 rather than at most 3.

Modifies tests and relevant mock data to conform to this change in endpoint behaviour.

In the process, adds a few development-related items:

• adds the django-extensions and werkzeug packages so that the development django container runs runserver_plus. This server provides a shell for debugging any time a view errors -- a useful development feature.
• add the types-requests package with type stubs for the requests package.

Closes #1424

[dchiller]

Tests are currently failing because we install the "production" version on the test runner but use the development docker-compose file (which now requires a development dependency for the runserver_plus command). I'll solve.

[dchiller]

Ok, I think it makes the most sense to replicate the production environment on the test runner... so in that case we'll need to use a different command for the django container on the test runner.