Add mutation to send password reset email

[kellenmace]

The intent of this mutation is to mimic WP's retrieve_password() function that runs when the "Lost Password" form is submitted: https://cloudup.com/cDFg7wCo1ig

I envision it working like this:

1. The user is presented with a form field in which they can enter their username or password.
2. When the form is submitted, the sendPasswordResetEmail mutation is used and accepts that username/password as an argument.
3. sendPasswordResetEmail gets the user, calls WP's get_password_reset_key(), which generates a password reset hash, stores it as the user_activation_key for that user in the database, then returns the key.
4. The user is sent an email with a password reset link that contains the reset key and their login, such as domain.com/wp-login.php?action=rp&key=12345&login=myusername. Both the email subject and body are filterable, though, so the password reset link can easily be filtered to point to the frontend URL for a headless WP site instead, for instance.

[kellenmace]

I submitted a PR for this feature.

Here is a gif showing the new feature in action –