[V3 i18n] Internationalise help for commands and cogs (#1143)

* Framework for internationalised command help

* Translator for class docstring of cog

* Remove references to old context module

* Use CogManagerUI as PoC

* Replace all references to RedContext

* Rename CogI18n object to avoid confusion

* Update docs

* Update i18n docs.

* Store translators in list instead of dict

* Change commands module to package, updated refs in cogs

* Updated docs and more references in cogs

* Resolve syntax error

* Update from merge

redbot/core/commands/commands.py

@@ -0,0 +1,74 @@
+"""Module for command helpers and classes.
+
+This module contains extended classes and functions which are intended to
+replace those from the `discord.ext.commands` module.
+"""
+import inspect
+
+from discord.ext import commands
+
+
+__all__ = ["Command", "Group", "command", "group"]
+
+
+class Command(commands.Command):
+    """Command class for Red.
+
+    This should not be created directly, and instead via the decorator.
+
+    This class inherits from `discord.ext.commands.Command`.
+    """
+
+    def __init__(self, *args, **kwargs):
+        self._help_override = kwargs.pop('help_override', None)
+        super().__init__(*args, **kwargs)
+        self.translator = kwargs.pop("i18n", None)
+
+    @property
+    def help(self):
+        """Help string for this command.
+
+        If the :code:`help` kwarg was passed into the decorator, it will
+        default to that. If not, it will attempt to translate the docstring
+        of the command's callback function.
+        """
+        if self._help_override is not None:
+            return self._help_override
+        if self.translator is None:
+            translator = lambda s: s
+        else:
+            translator = self.translator
+        return inspect.cleandoc(translator(self.callback.__doc__))
+
+    @help.setter
+    def help(self, value):
+        # We don't want our help property to be overwritten, namely by super()
+        pass
+
+
+class Group(Command, commands.Group):
+    """Group command class for Red.
+
+    This class inherits from `discord.ext.commands.Group`, with `Command` mixed
+    in.
+    """
+    pass
+
+
+# decorators
+
+def command(name=None, cls=Command, **attrs):
+    """A decorator which transforms an async function into a `Command`.
+
+    Same interface as `discord.ext.commands.command`.
+    """
+    attrs["help_override"] = attrs.pop("help", None)
+    return commands.command(name, cls, **attrs)
+
+
+def group(name=None, **attrs):
+    """A decorator which transforms an async function into a `Group`.
+
+    Same interface as `discord.ext.commands.group`.
+    """
+    return command(name, cls=Group, **attrs)