Fix data migrations for Reference.source

Due to a bug in the data migrations for the new Reference.source column introduced in #387 and #392, the start position for references created as a slice was used as both start and stop position in the new column for storing this data.

This commit fixes these migrations and on top of this adds a new migration which corrects any values set by the old migrations (in case they were run before this fix). The original columns for these values have not yet been dropped, so no data has been lost.

Thanks @ifokkema for reporting this issue.

Fixes #393 (closed)