Add a media/ext/ directory by default.
Add a media/ext/ directory by default.

We weren't providing the media/ext/ directory on master, which caused
Review Board to fail until it was provided. Now we create this directory
by default and just populate it with a .gitignore.

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