Changed login controller

Changed controller and view of login because username is now equal to
emailadress. This had broken the implementation of login. Login is now
fixed.
Changed gebruikers_model

Changed every instance of username into emailadress. See also changed
login implementation. More work on this class may be needed in the
future. We will make those changes when the errors present themselves.
Form validation admin view changed

Validation of adding user is complete. In the middle of writing
validation to change user also made minor change to the function that
create html code.