Copy JIRA attachments to Redmine attachments folder

The original script migrates the DB entries for attachments, but does not copy the actual files.

Because the bug IDs change during migration, and the attachment file naming scheme is completely different for Jira and Redmine, it is very difficult to copy the files manually. So I extended the script to do the copying.

Before running the migration, you should copy (or sym-link) your Jira attachments folder to `$REDMINE_HOME/jira_attachments`, so that the script can find them. This path is hard-coded as `JIRA_ATTACHMENTS_DIR` near the top of the script.