Handling other 400 errors as JSON

For some endpoints when receiving requests with a wrong format, Flask was returning a 400 response in HTML. This version handles those cases returning a JSON.

Storing invalid session IDs after logout

This version now stored the invalid session IDs after a user logs out, so clients can not use old session cookies to access protected routes

Version with seeded users and swagger page

This release includes:

• Seeded users so we don't need to manually sign up new users when the API starts
• A swagger page at /swagger to test the endpoints and learn about the API

Fully functional for API path

Merge pull request #2 from CodeSignal/dev

Updated documentation and storing docs in json files