Scaffolding fixes

I fix some issues in scaffolding templates which prevent generation of
controllers and views.
Database migration fixes

I add a missing database migration change set to change the column type
of sel_value.tax_rate from "decimal" to "double precision".

Furthermore, I fix some tab/space mismatches.
Add field templates for 1:n and m:n relations

I add view templates for one-to-many and many-to-many fields to the
grails-app/views/_fields folder.