WIP: Core functionality to allow a hiring manager to add applicant to CATS

This PR is meant to add this story: https://www.pivotaltracker.com/story/show/173413601

x-post from the link:

• Define candidate attributes
• Define Postgres DB schema (table, primary key etc.)
• Define env-specific postgres connection for local testing
• Define db creation: create cats DB for local testing
• Determine whether to store CVs in Postgres or elsewhere (disk or blob store)
• Define API endpoints to add/get a candidate and to list candidates
• Actually wire the API endpoint to the DB
• Replace atoms with mount based system start/stop
• Write model-level tests
• Write handler-level tests