FIX: Modal dialogs now truely center and work at all screen sizes (mobile)

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: