change column type to text for multi-permit columns

This affects sandbox tables and the shipments table (3 such fields in each). This is to allow uploading annual reports with lots of permits (a sample one saved in Dropbox for testing)

`rake db:migrate`

Note: there's duplication of SQL code in migration and in the helpers.sql file, which is because the migration needs access to some new functions; but the functions actually belong in the helper file, so it's just working around a non-perfect migration system.