Delete Slack workspace

[hacklschorsch]

There's a Slack workspace at ​https://tahoe-lafs.slack.com/ .

• It seems to have been generated in 2016-06-17.
• It has 38 members, but sees no usage / seems to be deserted.
• I can't see any messages since Slack doesn't show messages older than 90 days in this free version.
• I couldn't find any links to it from the trac wiki, and the only other tickets that mention it are #3721 and #2953; both don't seem to support keeping Slack up.

• Maybe it holds some old messages that would be nice to have?
• Maybe an admin can make an export of the channels?

I propose we archive or delete it.