New `about.json` fields (all optional):
- `authors`: An arbitrary string describing the theme authors
- `theme_version`: An arbitrary string describing the theme version
- `minimum_discourse_version`: Theme will be auto-disabled for lower versions. Must be a valid version descriptor.
- `maximum_discourse_version`: Theme will be auto-disabled for lower versions. Must be a valid version descriptor.
A localized description for a theme can be provided in the language files under the `theme_metadata.description` key
The admin UI has been re-arranged to display this new information, and give more prominence to the remote theme options.
