Add Captcha

[jackmcdade]

Our instance of Slackin got exploited this weekend and was used to spam invites for several thousand (at least) email addresses. A simple Captcha would have solved this.

[avatsaev]

try my fork see if it works, tested only on heroku, seemed to work fine

[jackmcdade]

Any update on this? I can't put my instance back online until there's bot protection.

[KrzysztofMadejski]

It has been implemented in #311 . Can be closed now.