Allow for different CAPTCHA providers

The current CAPTCHA system in JIRA is both very hard to decipher for end users, and not accessible.

It would be good if there was, as a minimum, an option to generate a new image on the fly without having to reload the page. That would improve usability but not solve the accessibility issues though.

It would be great if one could, for example through a plugin, swap the default CAPTCHA provider with another one. This could solve both readability and accessibility issues. The Recaptcha project at http://recaptcha.net/ — which was unfortunately dismissed without consideration in CONF-9026 — is an excellent candidate.

Note that this improvement request is valid for other Atlassian products (e.g. CROWD, CONFLUENCE...)