Disable debug toolbar unless 'debug' present on request

This change set makes the debug toolbar activate only if 'debug' is present on
the request, either as a URL parameter or a cookie. I find that the debug
toolbar, while useful, makes MediaWiki feel perceptibly more sluggish. This
seems like a good compromise.

Change-Id: I5b16cf6ab2bef9bac0c28d915e13d7adb7a26133
Merge "Disable debug toolbar unless 'debug' present on request"