R1:991bbb6242dd

Provide "builtin" files and use them to fix Pholio when files are deleted

Summary:
Fixes T3132. Currently, if a user deletes a file which is present in a mock, that mock throws an exception when loading. If the file is also the cover photo, the mock list throws an exception as well.

In other applications, we can sometimes deal with this (a sub-object vanishing) by implicitly hiding the parent object (for example, we can just vanish feed stories about objects which no longer exist). We can also sometimes…