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.

app/models/dmail.rb

@@ -146,7 +146,7 @@ class Dmail < ApplicationRecord
 
   def send_email
     if is_recipient? && !is_deleted? && to.receive_email_notifications?
-      UserMailer.dmail_notice(self).deliver_later
+      UserMailer.with(headers: { "X-Danbooru-Dmail": Routes.dmail_url(self) }).dmail_notice(self).deliver_later
     end
   end
 

app/models/user.rb

@@ -379,7 +379,7 @@ class User < ApplicationRecord
 
         if errors.none?
           UserEvent.create_from_request!(self, :email_change, request)
-          UserMailer.email_change_confirmation(self).deliver_later
+          UserMailer.with_request(request).email_change_confirmation(self).deliver_later
         end
       end
     end