Security: rename WEBHOOK_AUTHORIZATION --> WEBHOOK_SECRET

This fixes a low severity security issue affecting Anymail v0.2--v1.3.

Django error reporting includes the value of your Anymail
WEBHOOK_AUTHORIZATION setting. In a properly-configured deployment,
this should not be cause for concern. But if you have somehow exposed
your Django error reports (e.g., by mis-deploying with DEBUG=True or by
sending error reports through insecure channels), anyone who gains
access to those reports could discover your webhook shared secret. An
attacker could use this to post fabricated or malicious Anymail
tracking/inbound events to your app, if you are using those Anymail
features.

The fix renames Anymail's webhook shared secret setting so that
Django's error reporting mechanism will [sanitize][0] it.

If you are using Anymail's event tracking and/or inbound webhooks, you
should upgrade to this release and change "WEBHOOK_AUTHORIZATION" to
"WEBHOOK_SECRET" in the ANYMAIL section of your settings.py. You may
also want to [rotate the shared secret][1] value, particularly if you
have ever exposed your Django error reports to untrusted individuals.

If you are only using Anymail's EmailBackends for sending email and
have not set up Anymail's webhooks, this issue does not affect you.

The old WEBHOOK_AUTHORIZATION setting is still allowed in this release,
but will issue a system-check warning when running most Django
management commands. It will be removed completely in a near-future
release, as a breaking change.

Thanks to Charlie DeTar (@yourcelf) for responsibly reporting this
security issue through private channels.

[0]: https://docs.djangoproject.com/en/stable/ref/settings/#debug
[1]: https://anymail.readthedocs.io/en/1.4/tips/securing_webhooks/#use-a-shared-authorization-secret

docs/tips/securing_webhooks.rst

@@ -29,7 +29,7 @@ If you aren't able to use https on your Django site, then you should
 not set up your ESP's webhooks.
 
 
-.. setting:: ANYMAIL_WEBHOOK_AUTHORIZATION
+.. setting:: ANYMAIL_WEBHOOK_SECRET
 
 Use a shared authorization secret
 ---------------------------------
@@ -41,7 +41,7 @@ with webhook data, to prove the post is coming from your ESP.
 
 Most ESPs recommend using HTTP basic authorization as this shared
 secret. Anymail includes support for this, via the
-:setting:`!ANYMAIL_WEBHOOK_AUTHORIZATION` setting.
+:setting:`!ANYMAIL_WEBHOOK_SECRET` setting.
 Basic usage is covered in the
 :ref:`webhooks configuration <webhooks-configuration>` docs.
 
@@ -60,7 +60,7 @@ any of the authorization strings:
 
       ANYMAIL = {
           ...
-          'WEBHOOK_AUTHORIZATION': [
+          'WEBHOOK_SECRET': [
               'abcdefghijklmnop:qrstuvwxyz0123456789',
               'ZYXWVUTSRQPONMLK:JIHGFEDCBA9876543210',
           ],