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`