Make $wgCodeEditorEnableCore configurable per wiki

In preparation for enabling it for editing JS and CSS. The current plan
is to enable it in Beta Labs for testing as soon as bug 47235 is fixed,
and ideally to start turning it on for live sites next week.

Bug: 39653
Change-Id: I39c626d066797d887769c3715df88838e709d527
Merge "Make $wgCodeEditorEnableCore configurable per wiki"