T93008: Use {{DISPLAYTITLE}} or subpage title where appropriate.

We now work harder to find the best title for an article in the
generated PDF. We first use item.displaytitle if set; this would be
a user's explicit override. If that's not set, we use the value of
the {{DISPLAYTITLE}} magic word, if present. If that's not present,
we use the subpage title if subpages are enabled in the namespace and
this is not a single-item collection. Otherwise we fall back to the
item.title as before.

NOTE that this does not affect the page title for single-item
collections, since that comes from metabook.title, which is set
by the Collection extension.

Change-Id: I216eff064ee251d17020a5a4586a70149eaea431