Private hunts

[lavaloid]

Resolves #6

Changes made:

• New is_private field in the Hunt model
• Refactored organizer logic to the Hunt model
• Organizers will be able to use slug redirects as usual, but non-organizers or users who are not logged in will need to use the correct slug to access.
• Unit tests for private hunts
  • TODO: Currently, all of the tests pass but they log warnings, not sure how to fix this without silencing the error :/

We may also need a page to list all of a user's private hunt to prevent links from getting lost -- see #95