Ensure that Button Block is Represented in Subscription Emails

At the moment, our subscription emails do not display content created with the Button Block.

This makes sense for now, as the button is not an image, but HTML/CSS.

Since buttons are generally integral to the content of the post they are used in, perhaps we could at least display the linked URL in our subscription emails.

The issue names no file, test, or entry point. Start by locating the subscription-email rendering path and how Button Block content is handled, then identify existing email tests; done means the linked URL from a Button Block is represented in subscription emails and the behavior is covered by a test.

Written by the indexing model from the issue text.