3699 user password validation

This code adopted from https://github.com/otwcode/otwarchive/pull/1272 for Issue 3699 (https://code.google.com/p/otwarchive/issues/detail?id=3699).

It moves the LOGIN_LENGTH_MIN, LOGIN_LENGTH_MAX,PASSWORD_LENGTH_MIN and PASSWORD_LENGTH_MAX  constants out of the User controller and into the config.yml. I also refactored the Cucumber tests that cover user creation validation, and shortened some lines over 80 characters to facilitate reviewing here.