FIX: Modal dialogs now truly center and work at all screen sizes (mobile), now with working Bootbox

Sorry about the Bootbox modals not working before.

I plan on looking into getting it to also use the new way of displaying, but for now it displays using the old style.

Old pull request message:

Fixes modal dialogs, having them be centered vertically and horizontally, and allows them to work at any screen size by allowing scrolling if needed (perfect test: Notify dialog on iPhone 4).

Yes, the 4 divs for a dialog are needed:
 - One to define the "screen", to allow scrolling when the content gets too big/screen too small (#discourse-modal)
 - Two to define a "table" to allow it to be vertically aligned (#modal-outer-container and #modal-middle-container)
 - One finally to hold the modal dialog (#modal-inner-container)

Because this scales to smaller screen sizes, no additional styles are needed to make modals work on mobile :thumbsup: