Handle form response when performing email subscription

[Daksh14]

After entering your email, you are redirected to subscribe.php and there the response is displayed in plain text. Should handle the response properly and display it with the landing page.

[galegosimpatico]

I just had this today. //codidact.org/subscribe.php says:

Mail was not sent, please report the issue here

Where here is a hyperlink anchored in https://github.com/codidact/landing-page/issues

Added a report. #51