Add ID number to <div container in tpl for public facing online contribution and event pages

It's helpful for CSS or jQuery manipulation of forms to have a id# of the page located in the DOM. This can allow, for instance, the styling of pages 3, 5, and 10 easily from a single CSS file, rather than having to create extra.tpl files. id# in the DOM just makes things more flexible. I will be submitting a PR shortly.