Implement password policy

Problem/Motivation

Drupal does not enforce any password policy, you can create passwords consisting of only one letter.
This is very insecure and we should provide the possibility to add password policies and define a reasonably save one as default configuration.

Proposed resolution

Provide the possibility to define password constraints on

- length
- usage of characters (upper, lower, digits, special characters)
- how long the password is valid until it has to be changed
- already used passwords cannot be reused again

Default configuration should be:

- min length of 8 characters
- has to contain upper, lowercase, digit and special character
- passwords have to be changed every 6 month
- you cannot reuse passwords