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
closes https://github.com/TryGhost/Team/issues/1130
- migrated install theme process to new modal system and changed to install+activate
- added "Use theme" button that opens the new install+activate modal
- when the view theme screen opens the install modal, an `onSuccess` callback is passed which sets a property that will skip closing the install modal when the view theme modal is closed and transitions to the `settings.design` route leaving the "success" modal state on screen
- added a `skipErrors` option to `themeManage.activateThemeTask` so that it can be used from processes that already handle theme errors without opening extra modals on top
