NAS-132712 / 25.04 / Fix alert text format for Mattermost. (#15025)

* Mattermost does not render html code sent in
messages. This edit removes/replaces html code from
the message text and adopts the markdown formatting
used by Mattermost. As the message text is modified
only before sending, this fix will be immune to future
changes to the generic alert message formatting.

* Mattermost does not render html code.
This edit uses html2text to match approach used for the Slack, Telegram
and the other messaging services.

src/middlewared/middlewared/alert/service/mattermost.py

@@ -1,5 +1,7 @@
 import json
 import requests
+import html
+import html2text
 
 from middlewared.alert.base import ThreadedAlertService
 from middlewared.schema import Dict, Str
@@ -26,7 +28,7 @@ def send_sync(self, alerts, gone_alerts, new_alerts):
                 "username": self.attributes["username"],
                 "channel": self.attributes["channel"],
                 "icon_url": self.attributes["icon_url"],
-                "text": self._format_alerts(alerts, gone_alerts, new_alerts),
+                "text": html.escape(html2text.html2text(self._format_alerts(alerts, gone_alerts, new_alerts))),
             }),
             timeout=INTERNET_TIMEOUT,
         )