emails: include logging information in email headers.

Log the following information in email headers:

* X-Danbooru-User: the user's name and ID.
* X-Danbooru-IP: the user's IP.
* X-Danbooru-Session: the users' session ID.
* X-Danbooru-URL: the page that triggered the email.
* X-Danbooru-Job-Id: the ID of the background job that sent the email.
* X-Danbooru-Enqueued-At: when the email was queued as a background job.
* X-Danbooru-Dmail: for Dmail notifications, the link to the Dmail.
* X-Request-Id: the request ID of the HTTP request that triggered the email.

Also make it so we log an event in the APM when we send an email.

test/functional/dmails_controller_test.rb

@@ -147,6 +147,9 @@ class DmailsControllerTest < ActionDispatch::IntegrationTest
 
         assert_redirected_to Dmail.last
         assert_enqueued_emails 1
+
+        perform_enqueued_jobs
+        assert_performed_jobs(1, only: MailDeliveryJob)
       end
 
       should "not allow banned users to send dmails" do