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
no issue
- added details to the `marketplaceThemes` array in the `change-theme` controller
- added theme images obtained from https://ghost.org/themes/
refs https://github.com/TryGhost/Team/issues/480
- switched marketplace themes to a selection of Ghost's own free themes
- added install and preview buttons for all
- show theme screenshot in the install modal if it's a known theme
