Partly restore Mayhem's captcha fix

Destroy captcha after posting instead of reloading it.
Not strictly necessary since Google no longer blocks caching
but a good idea to reduce the number of captcha loads.

Keep captcha caching.
Recaptcha image URL parameter is no longer same as challenge

This fixes a bug where the captcha would load multiple times.
Recaptcha.reload("t") no longer works
generate QR thumbnails for .webm
wait for captcha image to exist before trying to check the URL
prevent event handlers from being called multiple times
fix captcha counting
more captcha fixes
update CHANGELOG
update setting description
Release 4chan X v1.7.23.