Update DiffResource to use allowed_item_mimetypes instead of allowed_mimetypes.
Update DiffResource to use allowed_item_mimetypes instead of allowed_mimetypes.

DiffResource kept its own list of allowed mimetypes, but it should have
just been using allowed_item_mimetypes. Switch to that instead. It also
helps resolve a conflict with some new code in Djblets.

chipx86/reviewboard,1tush/reviewboard,beol/reviewboard,custode/reviewboard,davidt/reviewboard,chipx86/reviewboard,brennie/reviewboard,beol/reviewboard,davidt/reviewboard,1tush/reviewboard,brennie/reviewboard,bkochendorfer/reviewboard,1tush/reviewboard,brennie/reviewboard,1tush/reviewboard,custode/reviewboard,reviewboard/reviewboard,1tush/reviewboard,davidt/reviewboard,reviewboard/reviewboard,KnowNo/reviewboard,1tush/reviewboard,custode/reviewboard,custode/reviewboard,KnowNo/reviewboard,bkochendorfer/reviewboard,davidt/reviewboard,KnowNo/reviewboard,bkochendorfer/reviewboard,bkochendorfer/reviewboard,sgallagher/reviewboard,1tush/reviewboard,chipx86/reviewboard,beol/reviewboard,brennie/reviewboard,beol/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,sgallagher/reviewboard,sgallagher/reviewboard,KnowNo/reviewboard,1tush/reviewboard,1tush/reviewboard,sgallagher/reviewboard