Channel IDs instead of names in document IDs

[stuartpb]

The current name-based document scheme is a huge problem if any of the channels in use get renamed. The system needs to be rolled back to using channel IDs internally, and only using names when displaying or interacting with the archive or reslackplan.

[stuartpb]

Due to the way that channels are laid out in exports... it's better to keep them by name, but make it so they have their channel ID attached as well, so the document can get "moved" if the channel's name changes.

[stuartpb]

Eh, considering that the plan around #12 and #13 involves putting everything into the DB on import anyway, I'd say it'd be better to just have channels replaced with their corresponding channel ID as part of that.

[stuartpb]

For the record, Slack moving to channel IDs in URLs was the big impetus for this change for me.