SpecialPage: DRY array filter for prefixSearchSubpages()

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