Adjust display to re-use elements introduced in #1577.

Proposed changes:

• Adds posts, followers, and following count to new follower email.
• Updates look-and-feel of the email.

Testing instructions:

• Apply this PR to a test site that's accessible from the internet.
• Go to Settings > Activitypub and make sure the New Follower notification setting is enabled.
• From a test account outside of your site, follow your user or your blog.
• Make sure the notification email looks as expected.

Before

Desktop

Desktop (no header)

Mobile

Changelog entry

• Automatically create a changelog entry from the details below.