Email should be required in mailing list subscription form

Problem/Motivation

Email is the only field in the mailing list subscription form, and it should be required. Submitting the form without entering an email raises an error:

Mailgun\Exception\InvalidArgumentException: Expected a different value than "". in Mailgun\Assert::reportInvalidArgument() (line 25 of /var/www/html/vendor/mailgun/mailgun-php/src/Assert.php).

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes