DEV: allow themes to render their own custom homepage (#26291)

This PR adds a theme modifier and route so that custom themes can opt to show their own homepage. See PR description for example usage.

spec/lib/homepage_helper_spec.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+RSpec.describe HomepageHelper do
+  describe "resolver" do
+    fab!(:user)
+
+    it "returns latest by default" do
+      expect(HomepageHelper.resolve).to eq("latest")
+    end
+
+    it "returns custom when theme has a custom homepage" do
+      ThemeModifierHelper.any_instance.expects(:custom_homepage).returns(true)
+
+      expect(HomepageHelper.resolve).to eq("custom")
+    end
+
+    context "when first item in top menu is no valid for anons" do
+      it "distinguishes between auth homepage and anon homepage" do
+        SiteSetting.top_menu = "new|top|latest|unread"
+
+        expect(HomepageHelper.resolve(nil, user)).to eq("new")
+        # new is not a valid route for anon users, anon homepage is next item, top
+        expect(HomepageHelper.resolve).to eq(SiteSetting.anonymous_homepage)
+        expect(HomepageHelper.resolve).to eq("top")
+      end
+    end
+  end
+end