Plupload: switch to urlstream upload method when the 'flash' runtime is used in non IE browsers. This ensures cookies are sent but limits the maximum file size that flash can handle.

By default only IE9 and older use flash so this change is mostly for completeness. It would affect only the (extremely rare) cases where a plugin disables the html5 runtime.
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Introduce a ms_site_not_found filter to replace NOBLOGREDIRECT.

Move some processing down below the point where we bail if there's no site. Add more documentation.

props jeremyfelt.
fixes #21143, see #27003.

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