Don't load the full entity into Lua to get labels/ sitelinks

This approach makes sure we have less round tripping between
PHP and Lua with large entity arrays, and reduces the
memory usage in Lua (especially if a lot of items are loaded).

In the future we might want to use a memcached label and sitelink
lookup to make things even faster.

Bug: 71743
Change-Id: Ic96dca7dc7abce339b1ce05d678db001fd432189
Merge "Don't load the full entity into Lua to get labels/ sitelinks"