FEATURE: Allow themes to specify modifiers in their about.json file (#9097)

There are three modifiers:
- serialize_topic_excerpts (boolean)
- csp_extensions (array of strings)
- svg_icons (array of strings)

When multiple themes are active, the values will be combined. The combination method varies based on the setting. CSP/SVG arrays will be combined. serialize_topic_excerpts will use `Enumerable#any`.

app/models/theme_modifier_set.rb

@@ -0,0 +1,69 @@
+# frozen_string_literal: true
+class ThemeModifierSet < ActiveRecord::Base
+  class ThemeModifierSetError < StandardError; end
+
+  belongs_to :theme
+
+  MODIFIERS ||= {
+    serialize_topic_excerpts: { combine_mode: :any, type: :boolean },
+    csp_extensions: { combine_mode: :flatten, type: :string_array },
+    svg_icons: { combine_mode: :flatten, type: :string_array },
+  }
+
+  validate :type_validator
+
+  def type_validator
+    MODIFIERS.each do |k, config|
+      value = public_send(k)
+      next if value.nil?
+
+      case config[:type]
+      when :boolean
+        next if [true, false].include?(value)
+      when :string_array
+        next if value.is_a?(Array) && value.all? { |v| v.is_a?(String) }
+      end
+      errors.add(k, :invalid)
+    end
+  end
+
+  after_save do
+    if saved_change_to_svg_icons?
+      SvgSprite.expire_cache
+    end
+    if saved_change_to_csp_extensions?
+      CSP::Extension.clear_theme_extensions_cache!
+    end
+  end
+
+  # Given the ids of multiple active themes / theme components, this function
+  # will combine them into a 'resolved' behavior
+  def self.resolve_modifier_for_themes(theme_ids, modifier_name)
+    return nil if !(config = MODIFIERS[modifier_name])
+
+    all_values = self.where(theme_id: theme_ids).where.not(modifier_name => nil).pluck(modifier_name)
+    case config[:combine_mode]
+    when :any
+      all_values.any?
+    when :flatten
+      all_values.flatten(1)
+    else
+      raise ThemeModifierSetError "Invalid theme modifier combine_mode"
+    end
+  end
+end
+
+# == Schema Information
+#
+# Table name: theme_modifier_sets
+#
+#  id                       :bigint           not null, primary key
+#  theme_id                 :bigint           not null
+#  serialize_topic_excerpts :boolean
+#  csp_extensions           :string           is an Array
+#  svg_icons                :string           is an Array
+#
+# Indexes
+#
+#  index_theme_modifier_sets_on_theme_id  (theme_id) UNIQUE
+#