Modernized version of SimpleModal

- remove all browser-specific hacks and references to specific browsers
- remove all positioning and dimension setting code
- remove all options to do with width/height control
- remove the `.simplemodal-wrap` element from HTML

By realizing these changes, it is possible to control modals entirely using CSS, without having to negate the styles that SimpleModal applies by default. Removing large blocks of code for legacy browsers makes SimpleModal even faster.

cc @rjmackay @dkobia