Fix for #2663: fix for original/96px-size avatars getting saved with bad permissions...

Fix for #2663: fix for original/96px-size avatars getting saved with bad permissions, causing failure to see images if background queues are different user from web server.

The original-size file is now forced to 0644 (all-readable), which should help. Not sure this is 100% ideal, but it's better than 0600!
(The other sizes were being created in other code and had sane read perms already.)

Watch out for similar issues in Twitter bridge etc; avatar import code should get cleaned up and consolidated.