fix slack signup link

[csweaver]

1. Fix the https://join-cellxgene-users.herokuapp.com/ app. Submitting an email triggers an error saying that the #cellxgene-users channel doesn't exist.

2. There are two different sign up links: https://join-cellxgene-users.herokuapp.com/ on README.md and https://join-cziscience-slack.herokuapp.com/ on docs/index.md and client/src/components/graph/graph.js

[neuromusic]

Hmm. Trying to figure out why join-cellxgene-users is broken.

Ideas:

1. It's a shared channel, so the slack API works differently
2. We've hit some limit based on signups vs number pf paid users: https://github.com/rauchg/slackin/blob/e41687b286c4dcaa24538383bd8a191d8e1b0871/app.json#L29

[neuromusic]

ok, toggled the config to add signups to #imaging and that works fine, so the problem appears to be with the shared channels...

[neuromusic]

issue added to slackin: rauchg/slackin#398

[neuromusic]

solution: get rid of channel signup.

[neuromusic]

fixed