Use CodeEditor for user css/js subpages

The test for $title->isCssJsSubpage() seems to have gotten lost in
the refactoring in r110794, so enabling CodeEditor for core only
affects the MediaWiki namespace. This restores that test, so user css/js
subpages also get CodeEditor when $wgCodeEditorEnableCore is true.

Change-Id: Id02825f922a1ed0aace7c9ffd940fe8d29bb5d79
Merge "Use CodeEditor for user css/js subpages"