Global setting for Parsoid HTTP proxy

On sites using $wgHTTPProxy, any curl requests made via MWHttpRequest
would be passed through that proxy. The Parsoid daemon is most probably
reachable directly, hence we want to be able to disable the proxy.
This can be done using the new $wgVisualEditorParsoidHTTPProxy which is
passed to MWHttpRequest in VisualEditor API.

Change-Id: I855249950b1b368ae0afe662154eb48e5b1a14f2