Refactor fuzzy help and clean up help command (#2122)

What's changed:
- Fixed issues mentioned on #2031
- Fuzzy help displays more like help manual
- Fuzzy help is easier and more flexible to use
- Fuzzy help string-matching ratio lowered to 80
- Help formatter is more extendable
- Help command has been optimized, cleaned up and better incorporates fuzzy help
- Added async_filter and async_enumerate utility functions because I was using them for this PR, then no longer needed them, but decided they might be useful anyway.
- Added `Context.me` property which is a shortcut to `Context.guild.me` or `Context.bot.user`, depending on the channel type.

redbot/core/commands/context.py

@@ -237,3 +237,20 @@ class Context(commands.Context):
             )
         else:
             return await self.send(message)
+
+    @property
+    def clean_prefix(self) -> str:
+        """str: The command prefix, but a mention prefix is displayed nicer."""
+        me = self.me
+        return self.prefix.replace(me.mention, f"@{me.display_name}")
+
+    @property
+    def me(self) -> discord.abc.User:
+        """discord.abc.User: The bot member or user object.
+
+        If the context is DM, this will be a `discord.User` object.
+        """
+        if self.guild is not None:
+            return self.guild.me
+        else:
+            return self.bot.user