Extend Modal to support in small viewports

[jugglinmike]

Improve support for modal dialog boxes in small (e.g. mobile) viewports:

• Ensure that dialogs never extend beyond the left-most and right-most boundaries of the viewport
• Enable vertical scrolling in cases where the dialogs' content extend beyond the bottom of the viewport
• Reposition the "dismiss" button to fall within the dialog's content area

@cbujara As we discussed on today's call, this partially addresses gh-143, specifically:

unable to resize/close instructions displayed on launch.

...but more work is necessary before we can call the "Supply and Demand of Cocoa" application "mobile friendly".

[cbujara]

Nice! Thanks for squeezing this one in, Mike.

[jugglinmike]

Sure thing!