fix(modal): backdrop z-index when stacking modals

Previously, backdrop wouldn't go over the previous open modal when a
modal is open on top of another.

Fixes #1653
Closes #1654
docs(typeahead): docs for typeahead-append-to-body

Closes #1663
fix(typeahead): correctly handle append to body attribute

Closes #1656
demo(all): fix demo scrolling issues

Fixes #1674
chore(ngVersion): update to 1.2.10