Standardize usage of **password** hash function

As preparation for introduction of a stronger hash function in https://github.com/piwik/piwik/issues/5728, this standardizes the usage of the current **password** hash function across the Login and UsersManager plugins. Specifically:

* Use UsersManager::getPasswordHash() throughout, instead of previous direct use of md5() in some places

* Concentrate the hash length sanity check in the newly created UsersManager::checkPasswordHash(). Previously, UsersManager\Auth and Login\PasswordResetter classes did it separately. Both checks were moved into the newly created UsersManager class function, as that is also where other public static password check functions reside.

* Replaced the "md5" string with "hash" in affected variable names and comments

No functional change is intended.