Expect assignments to come in with lowest due_times first
By default, order assignments by ascending due_time
Add tests for a new submission model
Create a new submission model in the database
Create a submission model and its relationships
Test for some basic submission validations
Add validations for the presence of author and assignment id to submissions
Test for users' and assignments' submissions method.
Test for a basic submission show page
Introduce submissions controller, and add routes and show view
Add paperclip to the Gemfile
Add paperclip's testing DSL
Add some tests for submission's source_code
Add a source code attachment to submissions in the DB
Add a source_code file in the submission_spec
Add tests for showing a submission's source attachment
Use 'shallow' resources in routes.rb
Add and validate for attachment presence
Display a submission's source code on #show
Add tests for a submission creation form