edit challenges and demos for forms sessions

The following changes have been made in order to merge HTML Forms, JS Forms, JS Forms 2 session into just two sessions:

JS Forms demo start and demo end have been modified to incorporate usage of the input event
JS Forms challenges handle-form-submit and checkbox-input have been edited to incorporate the input event / focusing elements / resetting forms
formvalidation challenge has been moved to html forms and the Readme has been edited

neuefische / web-exercises