Rewrite !populate command
Rewrite !populate command

Instead of pulling users into a challenge channel from the main channel,
!populate now works by taking a list of user names and inviting them
first into the main CTF channel and then into each challenge channel
that exists for that CTF.

In the event that there are no users to be invited, for example because
the users in the list are already present in the channel(s), no API call
to Slack will occur.

OpenToAllCTF/OTA-Challenge-Bot