Always allow licenseinfo to be run with a mention prefix (#5865)

Co-authored-by: Zephyrkul <23347632+Zephyrkul@users.noreply.github.com>

redbot/core/bot.py

@@ -1589,6 +1589,22 @@ class Red(
         """
         if not message.author.bot:
             ctx = await self.get_context(message)
+
+            # The licenseinfo command must always be available, even in a slash only bot.
+            # To get around not having the message content intent, a mention prefix
+            # will always work for it.
+            if not ctx.valid:
+                for m in (f"<@{self.user.id}> ", f"<@!{self.user.id}> "):
+                    if message.content.startswith(m):
+                        ctx.view.undo()
+                        ctx.view.skip_string(m)
+                        invoker = ctx.view.get_word()
+                        command = self.all_commands.get(invoker, None)
+                        if isinstance(command, commands.commands._AlwaysAvailableMixin):
+                            ctx.command = command
+                            ctx.prefix = m
+                        break
+
             if ctx.invoked_with and isinstance(message.channel, discord.PartialMessageable):
                 log.warning(
                     "Discarded a command message (ID: %s) with PartialMessageable channel: %r",