Login and user creation

[Wunst]

Most of the backend for our first sprint. Only thing missing now is users being able to change their passwords (#3). Also, we don't follow REST conventions yet. Pushing this back to the next sprint.

• Add dependency: express.js
• Use new ExpressJS server for login entry point
• Parse JSON request bodies
• Allow CORS access
• Add express-session boilerplate
• Save login session
• Add /logout entry point
• Fix status codes / redirects
• Make User.username unique index
• password hashing, create user, change password, reset password
• Delete empty file
• Allow insecure connections with credentials in development environment
• Add possibility of testing with an SQLite DB for development
• fix: Security: issues caused by queries with undefined parameters
• / entry point for session info