FEATURE: Support additional metadata in theme about.json (#6944)

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.

app/models/remote_theme.rb

@@ -3,6 +3,14 @@ require_dependency 'theme_store/tgz_importer'
 require_dependency 'upload_creator'
 
 class RemoteTheme < ActiveRecord::Base
+  METADATA_PROPERTIES = %i{
+                            license_url
+                            about_url
+                            authors
+                            theme_version
+                            minimum_discourse_version
+                            maximum_discourse_version
+                          }
 
   class ImportError < StandardError; end
 
@@ -16,6 +24,8 @@ class RemoteTheme < ActiveRecord::Base
     joins("JOIN themes ON themes.remote_theme_id = remote_themes.id").where.not(remote_url: "")
   }
 
+  validates_format_of :minimum_discourse_version, :maximum_discourse_version, with: Discourse::VERSION_REGEXP, allow_nil: true
+
   def self.extract_theme_info(importer)
     JSON.parse(importer["about.json"])
   rescue TypeError, JSON::ParserError
@@ -123,8 +133,12 @@ class RemoteTheme < ActiveRecord::Base
       end
     end
 
-    self.license_url = theme_info["license_url"]
-    self.about_url = theme_info["about_url"]
+    METADATA_PROPERTIES.each do |property|
+      self.public_send(:"#{property}=", theme_info[property.to_s])
+    end
+    if !self.valid?
+      raise ImportError, I18n.t("themes.import_error.about_json_values", errors: self.errors.full_messages.join(","))
+    end
 
     importer.all_files.each do |filename|
       next unless opts = ThemeField.opts_from_file_path(filename)