Added avatars to the forum postings.  Now users can upload a GIF or JPEG which is automatically scaled down so it's no bigger than 200*100 pixels.  They are also renamed to the user's name to ensure uniqueness.

Also, I've stopped storing a user's signature with their forum posts.  I did this because it was impractical to store the avatar with the post and figured I'd just keep the signature and avatar together.

Something to fix: stop GIFs losing their transparency when scaled down.