Conditionally import ModPythonRequest and WSGIRequest in the admin middleware.
Conditionally import ModPythonRequest and WSGIRequest in the admin middleware.

If Review Board is set up with WSGI without mod-python installed, it will throw
an error, even though it's not otherwise necessary. This change imports the
relevant request classes conditionally, and defines them as empty classes if
they can't import.

Fixes bug 2023.

chazy/reviewboard,chazy/reviewboard,beol/reviewboard,sgallagher/reviewboard,Khan/reviewboard,reviewboard/reviewboard,chazy/reviewboard,Khan/reviewboard,custode/reviewboard,sgallagher/reviewboard,KnowNo/reviewboard,sgallagher/reviewboard,1tush/reviewboard,chazy/reviewboard,atagar/ReviewBoard,chazy/reviewboard,beol/reviewboard,chipx86/reviewboard,davidt/reviewboard,1tush/reviewboard,Khan/reviewboard,1tush/reviewboard,1tush/reviewboard,beol/reviewboard,reviewboard/reviewboard,KnowNo/reviewboard,1tush/reviewboard,KnowNo/reviewboard,davidt/reviewboard,1tush/reviewboard,chazy/reviewboard,reviewboard/reviewboard,1tush/reviewboard,custode/reviewboard,chazy/reviewboard,atagar/ReviewBoard,Khan/reviewboard,chipx86/reviewboard,Khan/reviewboard,Khan/reviewboard,1tush/reviewboard,Khan/reviewboard,Khan/reviewboard,bkochendorfer/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,brennie/reviewboard,atagar/ReviewBoard,sgallagher/reviewboard,atagar/ReviewBoard,beol/reviewboard,atagar/ReviewBoard,atagar/ReviewBoard,chazy/reviewboard,atagar/ReviewBoard,custode/reviewboard,brennie/reviewboard,custode/reviewboard,1tush/reviewboard,chazy/reviewboard,atagar/ReviewBoard,chipx86/reviewboard,Khan/reviewboard,atagar/ReviewBoard,brennie/reviewboard,davidt/reviewboard,KnowNo/reviewboard,bkochendorfer/reviewboard,bkochendorfer/reviewboard,bkochendorfer/reviewboard,brennie/reviewboard,davidt/reviewboard