Adds empty session page

modified:   pizza.py
modified:   static/css/application.css
new file:   templates/no_active_sessions.jinja
Some small additions + enhancements

modified:   templates/active_sessions.jinja
modified:   templates/select_pizza.jinja
modified:   templates/welcome.jinja
Small touch - makes buttons equally sized

modified:   templates/helpers/pizza_helpers.jinja