Add support for a __NO_EDITOR__ token while bugs are fixed

[Flavien]

I really appreciate FCKeditor in MediaWiki, it's really increasing the adoption of MediaWiki in our enterprise.

I understand that it may take some time to get all the bugs fixed (e.g. <pre> and <any tag> support, [[Image:xyz|thumb]], etc) and I think that something like a __NO_EDITOR__ token would help. This would allow to keep FCKeditor enabled on the whole site, and for some tricky pages this token would allow experienced users to use pure wiki syntax bypassing the editor.

It should be in the spirit of:

$wgFCKEditorToken = "__USE_EDITOR__";

Maybe something like:

$wgFCKEditorDisableToken = "__NO_EDITOR__";

[alfonsoml]

dup of #1023