[WIP] Use CSS animations for modals

No issue

Every animation in Ghost should be dine 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

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Labeled as WIP because I'm not 100% positive if this is the best way to do things, from an Ember point of view.