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.

app/mailers/user_mailer.rb

@@ -5,7 +5,7 @@ class UserMailer < ApplicationMailer
   def dmail_notice(dmail)
     @dmail = dmail
     @user = dmail.to
-    mail(
+    mail_user(
       @user,
       from: "#{Danbooru.config.canonical_app_name} <#{Danbooru.config.notification_email}>",
       subject: "#{Danbooru.config.canonical_app_name}: #{dmail.from.name} sent you a message",
@@ -16,7 +16,7 @@ class UserMailer < ApplicationMailer
   # The email sent when a user requests a password reset.
   def password_reset(user)
     @user = user
-    mail(
+    mail_user(
       @user,
       from: "#{Danbooru.config.canonical_app_name} <#{Danbooru.config.account_security_email}>",
       subject: "#{Danbooru.config.app_name} password reset request",
@@ -27,7 +27,7 @@ class UserMailer < ApplicationMailer
   # The email sent when a user changes their email address.
   def email_change_confirmation(user)
     @user = user
-    mail(
+    mail_user(
       @user,
       from: "#{Danbooru.config.canonical_app_name} <#{Danbooru.config.account_security_email}>",
       subject: "Confirm your email address",
@@ -38,11 +38,20 @@ class UserMailer < ApplicationMailer
   # The email sent when a new user signs up with an email address.
   def welcome_user(user)
     @user = user
-    mail(
+    mail_user(
       @user,
       from: "#{Danbooru.config.canonical_app_name} <#{Danbooru.config.welcome_user_email}>",
       subject: "Welcome to #{Danbooru.config.app_name}! Confirm your email address",
       require_verified_email: false,
     )
   end
+
+  def dmca_complaint(to:)
+    @dmca = params[:dmca]
+    mail(
+      from: Danbooru.config.dmca_email,
+      to: to,
+      subject: "DMCA complaint",
+    )
+  end
 end