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>

Pipfile.lock

@@ -236,6 +236,13 @@
             ],
             "version": "==0.1.2"
         },
+        "schema": {
+            "hashes": [
+                "sha256:d994b0dc4966000037b26898df638e3e2a694cc73636cb2050e652614a350687",
+                "sha256:fa1a53fe5f3b6929725a4e81688c250f46838e25d8c1885a10a590c8c01a7b74"
+            ],
+            "version": "==0.6.8"
+        },
         "websockets": {
             "hashes": [
                 "sha256:0e2f7d6567838369af074f0ef4d0b802d19fa1fee135d864acc656ceefa33136",
@@ -597,6 +604,13 @@
             "markers": "python_version != '3.0.*' and python_version != '3.3.*' and python_version < '4' and python_version != '3.1.*' and python_version != '3.2.*' and python_version >= '2.6'",
             "version": "==2.19.1"
         },
+        "schema": {
+            "hashes": [
+                "sha256:d994b0dc4966000037b26898df638e3e2a694cc73636cb2050e652614a350687",
+                "sha256:fa1a53fe5f3b6929725a4e81688c250f46838e25d8c1885a10a590c8c01a7b74"
+            ],
+            "version": "==0.6.8"
+        },
         "six": {
             "hashes": [
                 "sha256:70e8a77beed4562e7f14fe23a786b54f6296e34344c23bc42f07b15018ff98e9",