Followup to r99653 (bug 31643) -- reenable client-side thumbnailing of SVGs on Specia...

Followup to r99653 (bug 31643) -- reenable client-side thumbnailing of SVGs on Special:Upload, with workaround for Firefox 7 regression.

Upstream firefox bug for hang loading some SVGs via data URI: https://bugzilla.mozilla.org/show_bug.cgi?id=694165
A fix may land as soon as Firefox 8.

This rev works around the bug by using window.URL.createObjectURL() instead of loading a data URI via FileReader; the object URL loads more cleanly and doesn't have the same hanging bug.

Needs to be replicated to UploadWizard (or moved to a shared lib!), some other fixes coming so no rush.