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
refs https://github.com/TryGhost/Ghost/issues/12608
requires https://github.com/TryGhost/Ghost/pull/12635
- adds `/settings/themes/install` route with `source` and `ref` query params that match the API. Shows a confirmation modal when accessed asking to confirm installation and activation
- does not allow Casper to be installed
- warns if installing the theme will overwrite an existing one
- follows similar process to zip uploads for error handling
- adds install/preview links for Massively in the free themes showcase
Co-authored-by: Sanne de Vries <sannedv@protonmail.com>
