Add setting 'Gallery.FileStorageDirectory' (#6569)

Author: stevenaw

src/NuGetGallery/Web.config

@@ -144,6 +144,7 @@
     <add key="Gallery.ExternalBrandingUrl" value=""/>
     <add key="Gallery.ExternalBrandingMessage" value=""/>
     <add key="Gallery.TrademarksUrl" value=""/>
+    <add key="Gallery.FileStorageDirectory" value="~/App_Data/Files"/>
     <add key="Gallery.EnforceDefaultSecurityPolicies" value="false"/>
     <add key="Gallery.RejectSignedPackagesWithNoRegisteredCertificate" value="false"/>
     <add key="Gallery.RejectPackagesWithTooManyPackageEntries" value="false"/>