Authentication: SignIn

[romshark]

Implement an email+password session-based authentication method. Implement an enablable root user with different permission modes (read-only | rw) for testing and beta-mode. The root user must be disabled in production!

• [x] API server: root user options
• [x] Store: CreateSession
• [x] GraphQL: mutations.SignIn
• [x] GraphQL: User.sessions
• [x] API test: mutations.SignIn
• [x] API test: mutations.SignIn (wrong email)
• [x] API test: mutations.SignIn (wrong password)
• [x] API test: mutations.SignIn (invalid input)
• [x] API test: User.sessions
• [x] Modules: Password hasher
• [x] Modules: Session key generator