settings: Migrate to stream_post_policy structure.

This commit includes a new `stream_post_policy` setting,
by replacing the `is_announcement_only` field from the Stream model,
which is done by mirroring the structure of the existing
`create_stream_policy`.

It includes the necessary schema and database migrations to migrate
the is_announcement_only boolean field to stream_post_policy,
a smallPositiveInteger field similar to many other settings.

This change is done to allow organization administrators to restrict
new members from creating and posting to a stream. However, this does
not affect admins who are new members.

With many tweaks by tabbott to documentation under /help, etc.

Fixes #13616.

zerver/openapi/python_examples.py

@@ -328,7 +328,7 @@ def update_stream(client, stream_id):
     # Update the stream by a given ID
     request = {
         'stream_id': stream_id,
-        'is_announcement_only': True,
+        'stream_post_policy': 2,
         'is_private': True,
     }