Support multiwiki metabook

[adamwight]

Currently, articles with an interwiki title are not parsed correctly by the render server. Wiki identity should be determined from the title, and the appropriate wikiconf loaded.

We can match the target article's base url against the siteinfo array, which saves us the trouble of parsing interwiki link syntax.

See also:

• https://gerrit.wikimedia.org/r/44848 (facilitates producing multiwiki books using the Collection extension)
• https://bugzilla.wikimedia.org/show_bug.cgi?id=17168