Prevent scrolling while modals are open

[erinknight242]

When modal windows are open but the contract definition is long enough for the main page to have a scroll bar, you are able to scroll the background behind the modal windows.

The main page body should have overflow: none while modal windows are open to prevent background scrolling (but scrolling within a modal window should still work.)

Currently, if the window is shorter than the modal height, you can scroll the background but not the modal window contents.