slack bridge: Add logic to prevent looping messages.

When using Slack Webhook integration to get messages from Slack to
Zulip, we don't want to send back messages from the Slack integration
bot.

This prevents that by filtering out any messages from the Slack Webhook
bots when sending messages from Zulip to Slack..

Fixes #825.

zulip/integrations/bridge_with_slack/run-slack-bridge

@@ -84,12 +84,18 @@ class SlackBridge:
             if w.startswith("@"):
                 zulip_msg["content"] = zulip_msg["content"].replace(w, "<" + w + ">")
 
+    def is_message_from_slack(self, msg: Dict[str, Any]) -> bool:
+        # Check whether or not this message is from Slack to prevent
+        # them from being tossed back to Zulip.
+        return msg["sender_email"] == self.zulip_config.get("email")
+
     def zulip_to_slack(self) -> Callable[[Dict[str, Any]], None]:
         def _zulip_to_slack(msg: Dict[str, Any]) -> None:
             slack_channel = get_slack_channel_for_zulip_message(
                 msg, self.zulip_to_slack_map, self.zulip_config["email"]
             )
-            if slack_channel is not None:
+
+            if slack_channel is not None and not self.is_message_from_slack(msg):
                 self.wrap_slack_mention_with_bracket(msg)
                 slack_text = SLACK_MESSAGE_TEMPLATE.format(
                     username=msg["sender_full_name"], message=msg["content"]