Added the migrate_user command

Migrate user is a command that lets you turn an existing user account
with another hasher into a django_pph-hash.
Addes tests for update_hash_threshold and update_hash_thresholdless

These functions are in charge of facilitating the migration, promotion
and demotion methods. They are now thoroughly tested.
Added test case for secret creation and verification functions

The test case covers the hash combination issues and detection of both,
valid secrets and invalid secrets.