Improvements

* moved sendgrid configuration from the outbound email settings page to a new extension specific page.
* got rid of the separate documentation page and include it in above.
* added SendGrid Configuration to the Mailings menu.
* disallow username/password when the extension directory is not writable; and therefore unable to create the .htaccess and .htpasswd files needed.
* added a new spam report template.
* modified the mail summary report template to include spam reports.
* webhook is ready to subprocess dropped notifications for unsubscribe, bounce, and spam reported once the proper identifying strings are known.