add stand-alone captcha, to block bots.

[komuw]

• https://github.com/flamego/captcha
• https://github.com/dchest/captcha

does not rely on google re-captcha or hcaptcha

[komuw]

see also:

• https://github.com/komuw/ong/issues/124 (seems complicated)
• https://github.com/komuw/ong/issues/95 (seems complicated)
• https://github.com/komuw/ong/issues/96 (is not fool proof)

Thus this issue might be the most viable.

[komuw]

If we do this, we might also have to use cookies. Because we could generate an image like 2 + 4 = ? and then the user's answer is submitted to a different server than the one that had generated the question.

[komuw]

We should also probably pair this with a hidden html form input, that if filled; we know this is a bot and fail.

[komuw]

https://stackoverflow.com/questions/38299930/how-to-add-a-simple-text-label-to-an-image-in-go