Wrap form around modal body and footer in order not have to use the input form attribute.

This MR is an fix for the bug described at #36 (closed) in which forms cannot be submitted in Internet Explorer and most versions of Edge.

Most modal forms had a submit button outside the form, that did submit the form with the form-attribute. This is not supported by Internet Explorer and only supported in the very last versions of Edge (which I doubt the University uses). For this reason, this MR moves the buttons back into the form. I consider this a better fix than having some kind of slowing down polyfill while we can simply do this without breaking anything.