server settings: Add setting for MAILING_ADDRESS.

The rules here are fuzzy, and it's quite possible none of Zulip's emails
need an address at all. Every country has its own rules though, which makes
it hard to tell. In general, transactional emails do not need an address,
and marketing emails do.

zerver/lib/send_email.py

@@ -41,6 +41,7 @@ def build_email(template_prefix, to_user_id=None, to_email=None, from_name=None,
         'realm_name_in_notifications': False,
         'support_email': FromAddress.SUPPORT,
         'email_images_base_uri': settings.ROOT_DOMAIN_URI + '/static/images/emails',
+        'physical_address': settings.PHYSICAL_ADDRESS,
     })
     subject = loader.render_to_string(template_prefix + '.subject',
                                       context=context,