Add CommandTree.active_guild_ids

[bijij]

Summary

Adds a new CommandTree.active_guild_ids property

Checklist

• [x] If code changes were made then they have been tested.
  • [x] I have updated the documentation to reflect the changes.
• [ ] This PR fixes an issue.
• [x] 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, ...)

[Rapptz]

Didn't commit this due to bikeshedding concerns and they still remain.