Add a button to request a new fancy captcha

This adds HTML, CSS, and JS to FancyCaptcha output to refresh the image,
which requests it via a new fancycaptchareload API.
It also cleans up the Asirra class a little.

Bug: 14230
Change-Id: I4e476f32de199534c9798fc78e8490b3ef91dd45
Merge "Add a button to request a new fancy captcha"