code cleanup. cached calculations in Markdown.Editor.js. Added option to set preview to manual. removed double timout application on refresh call. added timeout to init to refresh preview because loading the sanitizer is async.

added .wait handler to sanitizer script

Markdown.Editor cleanup. cached setScrollPanelTops calculations. Added option to set preview to manual, won't bind input events. removed double timeout call in refresh (applyTimeout + debounce). added wait callback in pagedown_editor to sanitizer script to refresh preview on load.
Merge branch 'pagedown_cleanup' of github.com:awbergs/discourse into pagedown_cleanup