add a means to clear the page cache on the CAPTCHA admin page

When you add CAPTCHAs on pages after they are cached, things won't work (e.g. see #632742: Caching with Boost, Varnish, ... causes "CAPTCHA validation error: unknown CAPTCHA session ID").

One solution is to clear the page cache during installation of the CAPTCHA module, but this is perhaps a bit drastic and it's not perfect either: there still can/will be caching between installation of the CAPTCHA module and final setup of the CAPTCHA module.

Better solution: add a button on CAPTCHA admin page to clear the page cache.