Implement Postgres repository layer

[darrensh3n]

• Adds Invite, Attempt, Answer, Submission, and SubmissionResult repositories
• Implements Postgres-backed repository layer using raw SQL
• Provides concrete implementations for assessments, questions, answers, submissions, and related entities
  Closesfleshing out repository layer #18

[adarshm11]

code looks good. before approval could you rebase? looks like the commit history has one of wayne's commits which was already merged to dev. remember, when rebasing, go onto dev branch and pull the latest from dev, then go back to your branch and run git rebase dev.