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`
- Blurring a post's title re-generates its slugPlaceholder, removing api
  spam on title change.
- 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
- PostModel's `generateSlug` now unwraps the slug from the JSONAPI wrapper
- PostModel's `generateSlug` has a hacky fix to prevent its changing its
  slug from "my-slug" to "my-slug-2".
- Added `slugPlaceholder` property to PostModel