Modal plugin initial commit

Plugin to build, show and hide modal dialogs from other plugins using the `build.wb-modal`, `show.wb-modal` and `hide.wb-modal` events.  

PR also includes:

1. Moved Magnific Popup `.mfp-*` CSS classes into the modal.scss file.
1. Converted session timeout plugin to use modal plugin.
1. Updated modal HTML to match lightbox modal HTML (discussion in #3213).
1. Added `<br>` to English session timeout message.