Use non-google captchas

[duxovni]

I'd like a way to run a matrix server without relying on Google for captchas. It'd be nice if synapse and other matrix servers made it easy to integrate self-hosted captcha systems like https://github.com/emotionLoop/visualCaptcha

[jomat]

I also don't want to pester my users with Google, that's why I don't use recaptcha and why I still hesitate to add my HS to the public server list.

[richvdh]

dup #1561