closes https://github.com/TryGhost/Ghost/issues/6458
- swap `ember-sortable` for `ember-drag-drop` in navigation UI
- extract PSM tag input into new `{{gh-psm-tags-input}}`
- add new `{{gh-token-input}}` that wraps `ember-power-select` and `ember-drag-drop` to replicate the previous selectize based tags input
- enhance `{{gh-psm-tags-input}}` behaviour to highlight selected primary tag and show "primary/internal" in selected tag titles
- 🔥 remove `selectize`
- 🔥 remove `jquery-ui`
- 🔥 remove unused `{{gh-navigation}}` component
