webhooks: Migrate 14 webhooks to use check_send_webhook_message.

These are the straightforward ones.

Note that there is a line in zerver.lib.test_classes.build_webhook_url
that lost test coverage. That's because most of our tests test using
stream messages so the webhook URLs being tested always have a query
parameter. So the line that accounts for there being no query
parameters never gets called, which is fine, but we should still
keep it.

zerver/lib/test_classes.py

@@ -669,7 +669,7 @@ class WebhookTestCase(ZulipTestCase):
 
         has_arguments = kwargs or args
         if has_arguments and url.find('?') == -1:
-            url = "{}?".format(url)
+            url = "{}?".format(url)  # nocoverage
         else:
             url = "{}&".format(url)