Reduce or eliminate bot signups

[mattlindsey]

We currently get hundreds of fake/bot signups per month. SendGrid only sends out emails to the first 100 people a month who join, so there may be real people who are not getting the welcome email with the link to Slack.

So, we should either improve our signup reCAPTCHA process somehow, or perhaps only allow signups via Github.

Requiring confirmation of email address (via Devise) would also eliminate a lot of fake accounts I think.