Avoid weak ref to local signal receiver in webhook tests.

Webhook tests define a local signal receiver function,
so connect it using the `weak=False` option to set
a good example.

(This isn't technically needed in the tests: the test receivers
are only connected while their definitions are still in scope,
so they couldn't possibly be garbage collected. But it doesn't
hurt, and it's good practice in case the test code gets copied.)

Also update the webhook docs to have a direct link to
Django's "listening to signals" info.

docs/usage/webhooks.rst

@@ -114,7 +114,11 @@ Webhook Notifications
 
 Once you've enabled webhooks, Djrill will send a ``djrill.signals.webhook_event``
 custom `Django signal`_ for each Mandrill event it receives.
-You can connect to this signal for further processing.
+You can connect your own receiver function to this signal for further processing.
+
+Be sure to read Django's `listening to signals`_ docs for information on defining
+and connecting signal receivers.
+
 Examples:
 
 .. code-block:: python
@@ -161,6 +165,8 @@ The available fields in the `data` param are described in Mandrill's documentati
 .. _Django signal: https://docs.djangoproject.com/en/stable/topics/signals/
 .. _inbound webhooks:
     http://help.mandrill.com/entries/22092308-What-is-the-format-of-inbound-email-webhooks-
+.. _listening to signals:
+    https://docs.djangoproject.com/en/stable/topics/signals/#listening-to-signals
 .. _sent-message webhooks: http://help.mandrill.com/entries/21738186-Introduction-to-Webhooks
 .. _whitelist/blacklist sync webooks:
     https://mandrill.zendesk.com/hc/en-us/articles/205583297-Sync-Event-Webhook-format