Re-enable MediaWiki.Usage.SuperGlobalsUsage.SuperGlobals sniff

Re-enable MediaWiki.Usage.SuperGlobalsUsage.SuperGlobals sniff

Disable it in specific files and places where there are legitimate uses
to access $_GET and $_POST directly.

For EditPage, which wants to output $_POST for debugging information,
introduce WebRequest::getPostValues() as a wrapper, matching the
existing ::getQueryValues().

Change-Id: I2cb0a7012fb7ed29dcd720056b42f56508ddc5fa