Introroduces order in journal migration.

This is so that attachments and custom fields are migrated when
workpackages are first touched. It satisfies the need for filenames
when connecting a work_package_journal with attachement_journals trough
attachable_journals, filename being a not null requirement on the
latter.
Adds support for empty changesets.

When just commenting, e.g., a work_package, one can omit all changes,
leading to an empty changed_data object. This is now supported.