(dev/mail#41) "Preview as HTML" - Fix error with tracking urls

[seamuslee001]

Overview

Fix regression in email preview. Use-case:

• Compose a "Mailing => New Mailing"
• Fill in basic in fields.
• In the body of the mailing, include a hyperlink
• Press the button "Preview as HTML"

5.12 port of #13956

ping @colemanw @eileenmcnaughton @monishdeb @mattwire

[civibot]

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[totten]

Updated both PR descriptions with clearer steps to reproduce.

Confirmed that this gets the pop-up to open. Relatedly, if I use the "Send Test" button, the generated test does produce a working/trackable link.

Interestingly, I noticed putting a hyperlink to www.google.com is problematic -- the preview renders well, but clicking the link leads to a white screen. This appears to be something unique to www.google.com, though -- other URLs (like en.wikipedia.org) seem fine. And it only affects the preview -- if "Send Test", all links were properly clickable. (Guess: google.com has some kind of anti-iframe mechanism?)