HTML e-mails for UserMailer (#6256)

- premailer gem to turn CSS into inline styles automatically
- rework UserMailer templates
- reword UserMailer templates

config/locales/en.yml

@@ -338,6 +338,7 @@ en:
       body: "%{reporter} has reported %{target}"
       subject: New report for %{instance} (#%{id})
   application_mailer:
+    notification_preferences: Change e-mail preferences
     salutation: "%{name},"
     settings: 'Change e-mail preferences: %{link}'
     signature: Mastodon notifications from %{instance}