emails: Move verbose_support_offers into a common context.

Server settings should just be added to the context in build_email, so that
the individual email pathways (and later, the email testing framework)
doesn't have to worry about it.

zerver/lib/notifications.py

@@ -404,7 +404,6 @@ def enqueue_welcome_emails(email, name):
     unsubscribe_link = one_click_unsubscribe_link(user_profile, "welcome")
     context = common_context(user_profile)
     context.update({
-        'verbose_support_offers': settings.VERBOSE_SUPPORT_OFFERS,
         'unsubscribe_link': unsubscribe_link
     })
     send_future_email(