Fixes and improvements to the Manage Preferences email

We just released a fix to the email which is sent to subscribers when they want to manage their subscription preferences.

Before the fix, the email heading didn't correctly include the status page name, instead showing a placeholder.

We also took this opportunity to rework the email copy to better align with the language of the subscriber journey. The email now uses the phrase "Manage Subscription" rather than "Sign-in" to better match the experience on the status page itself.