Unit tests for the create section functionality

This change adds unit tests for the create_section_form and create_section views
in the add people tool. It uses stubs and mocks extensively to minimize interaction
with the database or Django framework.

The tests do not test the actual Canvas API call, which is performed in
a separate utilities class. Ideally, when that class is fleshed out more there
will be more tests added here to include failure modes for the remote call.