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