Improve GibPotato direct messages if a person recieves a potato in a private channel they are not part of

[cleptric]

It gets awkward if a person receives a potato in a private they are not part of, as the link sent by the GibPotato bot opens the browser, with people ending up on the default channel.

We may look into the member's API to check if the mentioned person is indeed part of the channel.
https://api.slack.com/methods/conversations.members