Permissions redesign (#2149)

API changes:
- Cogs must now inherit from `commands.Cog` (see #2151 for discussion and more details)
- All functions which are not decorators in the `redbot.core.checks` module are now deprecated in favour of their counterparts in `redbot.core.utils.mod`. This is to make this module more consistent and end the confusing naming convention.
- `redbot.core.checks.check_overrides` function is now gone, overrideable checks can now be created with the `@commands.permissions_check` decorator
- Command, Group, Cog and Context have some new attributes and methods, but they are for internal use so shouldn't concern cog creators (unless they're making a permissions cog!).
- `__permissions_check_before` and `__permissions_check_after` have been replaced:  A cog method named `__permissions_hook` will be evaluated as permissions hooks in the same way `__permissions_check_before` previously was. Permissions hooks can also be added/removed/verified through the new `*_permissions_hook()` methods on the bot object, and they will be verified even when permissions is unloaded.
- New utility method `redbot.core.utils.chat_formatting.humanize_list`
- New dependency [`schema`](https://github.com/keleshev/schema)

User-facing changes:
- When a `@bot_has_permissions` check fails, the bot will respond saying what permissions were actually missing.
- All YAML-related `[p]permissions` subcommands now reside under the `[p]permissions acl` sub-group (tbh I still think the whole cog has too many top-level commands)
- The YAML schema for these commands has been changed
- A rule cannot be set as allow and deny at the same time (previously this would just default to allow)

Documentation:
- New documentation for `redbot.core.commands.requires` and `redbot.core.checks` modules
- Renewed documentation for the permissions cog
- `sphinx.ext.doctest` is now enabled

Note: standard discord.py checks will still behave exactly the same way, in fact they are checked before `Requires` is looked at, so they are not overrideable. 

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

redbot/core/commands/converter.py

@@ -0,0 +1,41 @@
+import re
+from typing import TYPE_CHECKING
+
+import discord
+
+from . import BadArgument
+from ..i18n import Translator
+
+if TYPE_CHECKING:
+    from .context import Context
+
+__all__ = ["GuildConverter"]
+
+_ = Translator("commands.converter", __file__)
+
+ID_REGEX = re.compile(r"([0-9]{15,21})")
+
+
+class GuildConverter(discord.Guild):
+    """Converts to a `discord.Guild` object.
+
+    The lookup strategy is as follows (in order):
+
+    1. Lookup by ID.
+    2. Lookup by name.
+    """
+
+    @classmethod
+    async def convert(cls, ctx: "Context", argument: str) -> discord.Guild:
+        match = ID_REGEX.fullmatch(argument)
+
+        if match is None:
+            ret = discord.utils.get(ctx.bot.guilds, name=argument)
+        else:
+            guild_id = int(match.group(1))
+            ret = ctx.bot.get_guild(guild_id)
+
+        if ret is None:
+            raise BadArgument(_('Server "{name}" not found.').format(name=argument))
+
+        return ret