[Permissions] Ability to set cog permissions for modrole and adminrole instead of only server specific roles

**Type of feature request**

Other

**Description of the feature you're suggesting**

Part of the available functionality of the bot is `[p]set addmodrole` and `[p]set addadminrole` to designate server specific roles as meta roles within the bot. This is useful since they're server agnostic, so if a bot is in multiple servers then any roles designated as `admin` or `mod` have the same permissions effect.

However, when using permissions to set command availability, it's currently only possible to designate them with `the ID, mention, or name of a server, channel, user or role which the bot can see`. This means that the permission tree needs to be configured for every server the bot is on, and it's not possible to set this up globally.

What I am proposing is the ability to add `modrole` and `adminrole` as options in the permissions cog.

```
[p]permissions setdefaultglobalrule deny play
[p]permissions addglobalrule allow play adminrole
```

This would allow anyone in a given server with a role designated as the adminrole the ability to use `play`, without having to allow it for every applicable role in every applicable server.