Actions like deleting realms may leave unreferenced uploads in the
attachment storage backend.
Fix these by walking the complete contents of the attachment storage
backend, and removing files which are no longer present in the
database. This may take quite some time, as it is necessarily O(n) in
the number of files uploaded to the system.
Enforcing a consistent `type` helps us double-check that we're not
playing fast-and-loose with any file paths for local files. As noted
in the comment, this is purely for defense-in-depth.
Passing `write_local_file` a consistent `type` requires removing the
"avatars" out of `realm_avatar_and_logo_path` -- which makes it
consistent across upload backends.
This, in turn, requires a compensatory change to zerver.lib.export, to
be explicit that the realm icons are exported from the avatars
directory. This clarity is likely an improvement.
Alex Vandiver