FEATURE: import phpBB avatars

This code adds all three avatar types to the import. Uploaded avatars and default gallery avatars are converted, hotlinked are pulled from the remote web site and stored as local. Beware, though: this is currently done during the importer run and can consume loong periods of time if many downloads fail with timeouts.

A minor fix concerns attachments to posts with white space in the real_name, this is handled properly now.
Merge pull request #2889 from JSey/patch-1

FEATURE: import phpBB avatars