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>

tox.ini

@@ -29,8 +29,9 @@ whitelist_externals =
 basepython = python3.6
 extras = docs, mongo
 commands =
-    sphinx-build -d "{toxworkdir}/docs_doctree" docs "{toxworkdir}/docs_out" -W -bhtml
-    sphinx-build -d "{toxworkdir}/docs_doctree" docs "{toxworkdir}/docs_out" -W -blinkcheck
+    sphinx-build -d "{toxworkdir}/docs_doctree" docs "{toxworkdir}/docs_out/html" -W -bhtml
+    sphinx-build -d "{toxworkdir}/docs_doctree" docs "{toxworkdir}/docs_out/linkcheck" -W -blinkcheck
+    sphinx-build -d "{toxworkdir}/docs_doctree" docs "{toxworkdir}/docs_out/doctest" -W -bdoctest
 
 [testenv:style]
 description = Stylecheck the code with black to see if anything needs changes.