New form styles

No issue, but references https://github.com/TryGhost/Ghost-UI/issues/36 for styles

- Additional classes in settings and invite user modal for the new form style
- Introduces a new way to do `<select>` elements, as shown in the invite user modal role select and theme select

Related styles are in Ghost-UI master as https://github.com/TryGhost/Ghost-UI/commit/fe2b7fd46c6f7ffcafd80af7f943ce9990f51941