Add captcha options

[ArendPeter]

(this came up during our meeting on 1/17/2022)

Election admins should be able to enable captchas to ensure that their votes are coming from humans and not machnies

There are many different captcha designs these days, let's going with one of the less annoying ones

[penyuan]

Thanks for opening this issue, I sincerely hope that we will at least commit to not using Google's reCaptcha!

Sorry I couldn't make the meeting, but would the captcha be a "modular" thing where whomever hosts the STAR server can drop in the solution they want to use? Or would it be hard-coded in?

I've been keeping track of Google reCaptcha replacements for a while, here are some that I am aware of:

• The list maintained by switching.software, which suggests:
  • Click Captcha
  • Securimage
  • Django Simple Captcha
  • Captcheck
• librecaptcha - Very actively developed, comes with multiple captcha generators plus a full REST API
• linklock - not strictly meant to be a captcha, but could be easily repurposed as one
• powcaptcha
• captchouli (looks very domain-specific, though)

Of the above, Securimage has a long, stable history with wide use (outside of Google reCaptcha), while librecaptcha seems to be the most complete and active project.

Hope this helps!