Refactored PostSettingsMenuController

Closes #2845. Depends on #2889, #2911. Ref #1351.
- Renamed `gh-blur-text-field` to `gh-blur-input`
- Changed title input in `editor.hbs` to use a `gh-blur-input`
- Added `regenerateSlugPlaceholderFlag` property `Post` model
- Blurring the post's title toggles the `regenerateSlugPlaceholderFlag`, removing api
  spam on title change (previously, regenerated on every change in title)
- Created `SlugGenerator` class to generify 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`