Refactored PostSettingsMenuController

Closes #2845.
Ref #1351.
- Refactored `PostSettingsMenuController` to appropriately set and display
  slug and publish date and their placeholders.
- Removed api spam on title change by putting `slugPlaceholder` generation
  inside of an `Ember.run.debounce` call.
- Renamed `gh-blur-text-field` to `gh-blur-input`
- Created `SlugGenerator` class to abstract slug generation.
- Added `timestampVerification` function to `utils/date-formatting`
- `utils/date-formatting` now uses `strict` parsing of dates
- Added more acceptable date formats to accommodate strict parsing
- Moved `isDraft` and `isPublished` computed properties from
  `EditorController` to `PostModel`
Merge pull request #2900 from novaugust/ember-settings-menu-cherry-picked

Rebuild Post Settings Menu
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
- Clear markers when codemirror is destroyed
- make Editor subcomponents communicate through the Editor Controller
- Set Codemirror and html preview shared scrolling
- Set CodeMirror, html preview css scroll class with util
- Create 'scratch' property in Editor controller; prevents a model save wiping image markers due to markdown bindings
- Add editor and html preview actions to handle img upload start/finish
- disable codemirror when an image is being uploaded, enables on success or failure
- Fix editor wordcount when there are 0 words
- Add modal dialog when transitioning out of the editor with an unsaved post
- Add window.onbeforeunload handling with `.unloadDirtyMessage()` on editor controller
- and various other things