SpecialPage: DRY array filter for prefixSearchSubpages()

Everytime this gets duplicated it isn't immediately obvious what
all the nested array_slice/preg_quote stuff does. Having it once
place removes the need to figure it out again or look at it as
new code (and ensures they're in sync and saves maintenance).

Change-Id: Iefe340729a55c9bb52a4931310966d0f33041205
Merge "SpecialPage: DRY array filter for prefixSearchSubpages()"