Created self.find_submission_and_status assignment model method

Moved logic relating to finding a student user's submission and that submission's status. Returns the completed submission. If none exists then @completed_submission is nil.
Created Assignment.create_submission_data

Moved controller logic for submission and submission placeholder creation into Assignment model. Logic is held in self.create_submission_data which returns a hash containing the submission object and the submission title and content placeholders respectively. Placeholders will populate the editor if the submission has not been marked complete.
Refactored Assignment model's self.find_submission_and_status method

No change in functionality, but the refactored version is more readable and clean.
Refactored Assignment model's self.create_submission_data method

Also made some small changes self.find_submission_and_status for readability and Ruby convention.
Merge pull request #62 from whoisglover/assignment_logic_refactor

Assignment logic refactor