Allow public signup for our biothings slack workspace

[newgene]

We currently have a biothings.slack.com available, but this is by invitation only as slack currently does not provide open signup. There is some workarounds available, like those mentioned in this post:

https://www.quora.com/Is-there-a-way-to-enable-anyone-to-join-a-Slack

This slackin option looks a good one to give it a try.

[newgene]

Just noticed that Slack now allow generating a sharable invitation link (can be set to "never expire") for new users to sign up. If that works well, we probably don't need to do anything further.

https://get.slack.help/hc/en-us/articles/201330256-Invite-new-members-to-your-workspace

Will keep this issue open for now till this is verified working well.