Allow admins to delete private streams (backend only).

This is the backend piece.  Getting the UI right here is a bit
more complicated here, but this allows admins to use the API
to delete streams.

zerver/lib/streams.py

@@ -11,6 +11,26 @@ from zerver.models import UserProfile, Stream, Subscription, \
     Realm, Recipient, bulk_get_recipients, get_recipient, get_stream, \
     bulk_get_streams
 
+def access_stream_for_delete(user_profile, stream_id):
+    # type: (UserProfile, int) -> Stream
+
+    # We should only ever use this for realm admins, who are allowed
+    # to delete all streams on their realm, even private streams to
+    # which they are not subscribed.  We do an assert here, because
+    # all callers should have the require_realm_admin decorator.
+    assert(user_profile.is_realm_admin)
+
+    error = _("Invalid stream id")
+    try:
+        stream = Stream.objects.get(id=stream_id)
+    except Stream.DoesNotExist:
+        raise JsonableError(error)
+
+    if stream.realm_id != user_profile.realm_id:
+        raise JsonableError(error)
+
+    return stream
+
 def access_stream_common(user_profile, stream, error):
     # type: (UserProfile, Stream, Text) -> Tuple[Recipient, Subscription]
     """Common function for backend code where the target use attempts to