Local authentication with passport

  - Adds local authentication through a User mongoose model
  - Not hooked up with home page (to avoid conflicts with changes there)
  - Usage:
    - /login - logs the user in
    - /logout - logs the user out
    - /profile - shows the user profile (so far just shows username)
  - There is a script to seed one user at /files/seed_users.js

  - TODO: user admin and social login