Rolling back part of the shouldExtendBlock optimization from earlier. The optimization was too aggressive, and it started breaking a media query test. The new code is much more conservative in when it will skip performing the extend. It will only skip a block that has no statements in it - excluding other rulesets, which will get extended themselves.
Put back in the hoistable check since it fixes a handful of diffs in our sass.
Merge remote-tracking branch 'origin/feature/extend-support' into feature/printable-refactor
Updates to isPrintable based on test failures.