[Bugfix] Links missing from Mattermost notification

Notification loses its links when sent through Mattermost.

Expected:

Result:

Most likely related to how the message is stripped of HTML and converted to Markdown. In /processors/mattermost/processor.py:

def process(self, **kwargs):
        """Process the message and send a mattermost notification."""
        logging.debug("%s - status:running - kwargs:%r", self, kwargs)

        device_token = kwargs["device_token"]
        email = kwargs["email"]
        device_id = kwargs["device_id"]
        subject = f'[{kwargs["channel_name"]}] - {kwargs["summary"]}'
        content_without_styles = Cleaner(style=True).clean_html(kwargs["message_body"])
        message_body = MarkdownConverter().convert_soup(BeautifulSoup(content_without_styles, "lxml"))