FEATURE: Theme settings (2) (#5611)

Allows theme authors to specify custom theme settings for the theme. 

Centralizes the theme/site settings into a single construct

lib/theme_settings_parser.rb

@@ -0,0 +1,66 @@
+class ThemeSettingsParser
+  class InvalidYaml < StandardError; end
+
+  def initialize(setting_field)
+    @setting_field = setting_field
+    @types = ThemeSetting.types
+  end
+
+  def extract_description(desc)
+    return desc if desc.is_a?(String)
+
+    if desc.is_a?(Hash)
+      default_locale = SiteSetting.default_locale.to_sym
+      fallback_locale = desc.keys.find { |key| I18n.locale_available?(key) }
+      locale = desc[I18n.locale] || desc[default_locale] || desc[:en] || desc[fallback_locale]
+
+      locale if locale.is_a?(String)
+    end
+  end
+
+  def create_opts(default, type, raw_opts = {})
+    opts = {}
+    opts[:description] = extract_description(raw_opts[:description])
+
+    if type == @types[:enum]
+      choices = raw_opts[:choices]
+      choices = [] unless choices.is_a?(Array)
+      choices << default unless choices.include?(default)
+      opts[:choices] = choices
+    end
+
+    if [@types[:integer], @types[:string], @types[:float]].include?(type)
+      opts[:max] = raw_opts[:max].is_a?(Numeric) ? raw_opts[:max] : Float::INFINITY
+      opts[:min] = raw_opts[:min].is_a?(Numeric) ? raw_opts[:min] : -Float::INFINITY
+    end
+    opts
+  end
+
+  def load
+    return if @setting_field.value.blank?
+
+    begin
+      parsed = YAML.safe_load(@setting_field.value)
+    rescue Psych::SyntaxError, Psych::DisallowedClass => e
+      raise InvalidYaml.new(e.message)
+    end
+    raise InvalidYaml.new(I18n.t("themes.settings_errors.invalid_yaml")) unless parsed.is_a?(Hash)
+
+    parsed.deep_symbolize_keys!
+
+    parsed.each_pair do |setting, value|
+      if (type = ThemeSetting.guess_type(value)).present?
+        result = [setting, value, type, create_opts(value, type)]
+      elsif (hash = value).is_a?(Hash)
+        default = hash[:default]
+        type = hash.key?(:type) ? @types[hash[:type]&.to_sym] : ThemeSetting.guess_type(default)
+
+        result = [setting, default, type, create_opts(default, type, hash)]
+      else
+        result = [setting, value, nil, {}]
+      end
+
+      yield(*result)
+    end
+  end
+end