Improve uncached page rendering times

[rjackson]

There are a few areas of the wiki where edits or cache purges takes double-digit seconds, and for a few of our larger pages over a minute – long enough for the load balancer serve a time out to editors.

In a98be18 I've managed to roughly half page load times for cached pages from around ~1.5 to .75s (unscientific measurements, mind), by swapping from Scribunto's default Lua parsing to their PHP extension. That doesn't appear to have made much of a difference to uncached pages however – perhaps that was a fairly fixed overhead, rather than one that scales with page size.

Some useful pages for testing:

Test case	Page	Page length	Purge load time
Simple page	Voice_actors	101	1.076s
Typical page	Scout	37290	11.872s
Large page	User:Gabrielwoj/Images/Decorated	2377186	60s+ (timeout)

Purge load time: Average of 5 loads from the live wiki, at time of writing