XWIKI-6882 : Improvements on user changing password. Move back to passwd.vm

After discussing it with Vincent, we agreed that it was a better solution to keep the password change feature in passwd.vm instead of doing it through the ChangePassword page I introduced. 
So I moved back my code to passwd.vm and changed the method to check the password, so it is accessible from the template.
It should solve the last issues with the functional tests.