Transition to editor.edit after saving a new post

Closes #2860
- `editor-base-controller`'s `save` action returns a promise with the model
  after saving
- `EditorNewController` will transition to `EditorEditController` upon a
  successful save (model has an id)
- Removed a console.log in editor's save function
Merge pull request #2930 from novaugust/save-new-post-transition

Add transition to editor/:id upon saving new post
closes #2426, closes #2781, closes #2913
- Concatenate vendor files on change of js in core/shared/
- Editor subcomponents communicate through editor controller
- 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(), returns an object with two keys: one with the markdown string with markers, the other without
- Set CodeMirror, markdownPreview css scroll class with util
- Create `scratch` prop in editor, prevents a save on the model from wiping out image markers due to markdown bindings
- Clear markers when codemirror is destroyed
- Editor controllers' `isDirty` checks `scratch` !== `markdown`, and the model's `isNew` and `isDirty` properties
- Add modal dialog alert when transitioning out of the editor with an unsaved post
- Move editor htmlPreview image upload handler to EditorControllerMixin
- Add editor and markdown-preview actions to handle image upload start/finish
- disable codemirror when an image is being uploaded, re-enable on success or failure
- and various other things