Set up a listing page for /api/ in all projects
This patch is following the implementation of the front page templates by
calling extract2.php in an Apache RewriteRule. This keeps such portals
together, and avoids adding more complexity to Varnish.
Depends on https://gerrit.wikimedia.org/r/#/c/228429/, which adds
API_listing_template to the allowed templates in extract2.php.
Bug: T107086
Change-Id: I746dd85aedee91bc2cbb8bd7ada5b2bfdf0e2228