About this PR:

• New regex in core/utils.py.
• Add a parse_channel_topic function that uses the new regex to parse the channel topic and return a tuple of 'title', 'user ID', and 'other recipients IDs'.
• Remove couple of old regexes as they're no longer used.

With these new regex and methods, it would be easier to check and match the recipient ID.

Currently, in core/thread.py, line 1243:

channel = discord.utils.find(
                lambda x: str(recipient_id) in x.topic if x.topic else False,
                self.bot.modmail_guild.text_channels,
            )

..the bot just checks if the user ID is in the channel topic, regardless the string format. This PR solves that.

Please check.