Implement (bot_)has_guild_permissions

[XuaTheGrate]

Summary

Adds two new decorators for checking guild wide permissions, as opposed to the current has_permissions decorator which runs on the invocation channel permissions.
This allows for better checking of certain permissions, such as mute_members for voice, and manage_roles which is different for channel overrides.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[yo-ru]

I'm glad you did this.
Will make things more simplistic.
+1

[Rapptz]

This implementation seems to work with a DM context but I feel like the name itself implies guild_only so I don't think working in DM is a desirable feature for these particular checks.