[SECURITY] Bug 263780: Exporting a bug to XML exposes user comments and attachment...

[SECURITY] Bug 263780: Exporting a bug to XML exposes user comments and attachment summaries which are marked as private to users who are not members of the group allowed to see private comments and attachments.  XML export is not exposed in the user interface, but is available to anyone who knows the correct URL to invoke it.  This only affects sites that use the 'insidergroup' feature.
Patch by Joel Peshkin <bugreport@peshkin.net>
r=vladd,justdave, a=justdave