Argon2 for user password hashing

Argon2 is a modern password hashing algorithm. It is significantly more secure than the previous algorithm (salted SHA512). User logins with Argon2 are relativly slow and cause significant spikes in CPU and memeory (100MB) usage.

Existing passwords are gradually migrated to Argon2 on login.

Closes #81 (closed).

To be merged after !107 (merged). Should be merged into master however.