Better Error Message for "Follow" Button on Testimony Detail Page

[Mephistic]

Describe the bug
On the Testimony Detail page, a logged out visitor can click the follow bill button (e.g. "Follow H.54"). Following only makes sense for a logged-in user (from both a product and code standpoint), so this button will quietly do nothing when clicked.

To Reproduce
Steps to reproduce the behavior:

1. Go to a Testimony Detail page (e.g. https://maple-dev.vercel.app/testimony/5PO5TI8lWo-dB8_rmKyWT/1 ) when logged out
2. Click on 'Follow H.54'
3. See that nothing happens

Expected behavior

A logged-out visitor should be directed to the login flow when they click the Follow button (and redirected back to the Testimony Detail page after signing in - similar to when a logged-out visitor visits another auth-protected page like the Newsfeed)