refs https://github.com/TryGhost/Team/issues/1149
- added Casper to the hardcoded official themes list with a ref of `'default'` so that the install theme process knows to process it differently
- updated the install-theme modal
- removed the `willOverwriteDefault` getter because it's a path that can't be reached and is now handled differently
- changed `installThemeTask` to only perform an activation when a default theme is passed in
