Use CSS animations for modals

No issue

Every animation in Ghost should be done with CSS where possible, so this PR adds them to modals.

- Changes Modals to use CSS-based animations
- All modals have an animation, so the option is removed from the handlebars helper
- Removes `animation` from the handlebars helper, as all had one anyway
- Removed the `centered` style - All modals should be left-aligned
- Buttons now default to being on the left
- Cleans up some styles (modal body top spacing) which #4632 helped clean up