Add DMCA complaint form.

Add a form for submitting DMCA complaints. The complaint is emailed to
the site owner, and a confirmation email is sent to the submitter.

test/functional/dmcas_controller.rb

@@ -0,0 +1,28 @@
+require "test_helper"
+
+class DmcasControllerTest < ActionDispatch::IntegrationTest
+  context "show action" do
+    should "work for anonymous users" do
+      get dmca_path
+      assert_response :success
+    end
+  end
+
+  context "create action" do
+    should "work" do
+      dmca = {
+        name: "John Doe",
+        email: "test@example.com",
+        address: "123 Fake Street",
+        infringing_urls: "https://example.com/1.html\nhttps://example.com/2.html",
+        original_urls: "https://google.com/1.html\nhttps://google.com/2.html",
+        proof: "source: me",
+        signature: "John Doe",
+      }
+
+      post dmca_path, params: { dmca: dmca }
+      assert_response :success
+      assert_emails 2
+    end
+  end
+end