HH-37938 fix some tornado3 incompatibilities and bugs
HH-37938 move handler.py methods around
HH-37938 add early, late and template postprocessors
HH-37938 add some docs
HH-37938 update some old tests, add postprocessing tests, fix None in Placeholder
HH-37938 fix debugging production errors and try..except late postprocessors
Merge remote branch 'origin/HH-37938' into release-candidate
release 2.18.1