(Bug 4860) Pre-fetch textcaptcha items and store them in the DB

We should be nice to both our users and the textcaptcha service. We
pre-fetch several hundred items with a 1-second delay so we don't hammer
the textcaptcha service with too many requests too close together at
active hours.

When we need a captcha, we check our DB first. If we run out, then we
do a remote fetch. This should also make the captcha show up more
quickly to users.
Merge pull request #243 from afuna/textcaptcha-cache-bug4860

(Bug 4860) Pre-fetch textcaptcha items and store them in the DB