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`.

spec/models/remote_theme_spec.rb

@@ -37,6 +37,9 @@ describe RemoteTheme do
               "love": "#{love_color}",
               "tertiary-low": "#{tertiary_low_color}"
             }
+          },
+          "modifiers": {
+            "serialize_topic_excerpts": true
           }
         }
       JSON
@@ -85,6 +88,8 @@ describe RemoteTheme do
       expect(remote.theme_version).to eq("1.0")
       expect(remote.minimum_discourse_version).to eq("1.0.0")
 
+      expect(@theme.theme_modifier_set.serialize_topic_excerpts).to eq(true)
+
       expect(@theme.theme_fields.length).to eq(9)
 
       mapped = Hash[*@theme.theme_fields.map { |f| ["#{f.target_id}-#{f.name}", f.value] }.flatten]

spec/models/theme_modifier_set_spec.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+require 'rails_helper'
+
+describe ThemeModifierSet do
+  describe "#resolve_modifiers_for_themes" do
+    it "returns nil for unknown modifier" do
+      expect(ThemeModifierSet.resolve_modifier_for_themes([1, 2], :unknown_modifier)).to eq(nil)
+    end
+
+    it "resolves serialize_topic_excerpts correctly" do
+      t1 = Fabricate(:theme)
+      t1.theme_modifier_set.update!(serialize_topic_excerpts: true)
+      t2 = Fabricate(:theme)
+      t2.theme_modifier_set.update!(serialize_topic_excerpts: false)
+
+      expect(ThemeModifierSet.resolve_modifier_for_themes([t1.id, t2.id], :serialize_topic_excerpts)).to eq(true)
+
+      t1 = Fabricate(:theme)
+      t1.theme_modifier_set.update!(serialize_topic_excerpts: nil)
+
+      expect(ThemeModifierSet.resolve_modifier_for_themes([t1.id, t2.id], :serialize_topic_excerpts)).to eq(false)
+    end
+  end
+end