Use the static storage to check mimetype icon existence.
Use the static storage to check mimetype icon existence.

Our mimetype icon existence code was assuming the existance of
STATIC_ROOT and then looking for file existence from there. This isn't
really how it should be used, and will fail when it's not even local
storage or when it's a dev install.

Now, we use the pipeline storage's exists() function, which will do the
right thing. As part of this, I've cleaned up the path building a little
bit.

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