Reach Editor parity with Ember

closes #2426, closes #2781, closes #2913
- Concatenate vendor files on change of js in core/shared/
- Add all the markerManager stuff to its own mixin
- make markers a shared object for all that mix it in. makes it easier to use helper functions in different modules
- add getMarkdown method, returns object with two keys holding the markdown: one with markers, the other without
- set up shared scrolling between codemirror and html preview
- make Editor subcomponents communicate through the Editor Controller
- Create 'scratch' property in Editor controller; prevents a model save wiping image markers due to markdown bindings
- Clear markers when codemirror is destroyed
- Add editor and markdown-preview actions to handle img upload start/finish
- disables codemirror when an image is being uploaded, enables on success or failure
- Fix editor wordcount for 0 words
- Add `.unloadDirtyMessage()` to editor controller
- Add modal dialog when transitioning out of the editor with an unsaved post
- Add window.onbeforeunload handling
- and various other things