Emails are not respecting the direction of a user's locale

For sites that allow users to set their locale, if the user’s locale has a different text direction than the site’s default locale, that locale’s text direction is not being applied to emails sent from Discourse. This is a regression from how Discourse handled user locales in emails in 2016: Emails for rtl languages have ltr direction.

We are still setting the local to the user’s effective_locale in the send_notification_email method, but it is not being used to set the email’s text direction.