Backend API for working with user votes in the database

[eritbh]

Ref #9, blocked on #22

Planned API routes:

• GET /api/votes/:category_id - Retrieve all votes for the current user in a given category (requires user authentication)
• POST /api/votes/:category_id - Write a list of votes to the database and remove all other votes for this user and category (requires user authentication)
• GET /api/vote-totals/:category_id - Get vote counts from all votes in a category; does not include individual vote data, reddit usernames, etc. (requires host authentication)