bilibili: use another user agent

[zImPatrick]

it seems that they're specifically allergic against user agents that start with Mozilla/ OR include the word github (that rules out the generic UA and the cobalt-specific UA in config.js), so I've just chosen to use facebookexternalhit/1.1 (+http://www.facebook.com/externalhit_uatext.php)

(also, the bilibili.com link with part id test seems to be broken because the video it depends on seems to be gone?)