Fix password length and occasionally wrong urls in password reset mail

* Set length for generated passwords to 8. [Relevant](https://github.com/plone/Products.CMFPlone/blob/master/Products/CMFPlone/RegistrationTool.py#L297-L298)
* The url for the password reset mail was generated by string concatenation with the `userid`. This fails for instance if the user has a `-` in his id: `user@exam-ple.ch` -> `user-40exam--ple.ch`
This fix changes that to `absolute_url` on the users homefolder.

@maethu