This adds a data migration to fix those cases. It defaults to showing
1 unit of the specified weight. That is, if the user chose Kg, it'll
display 1 as unit.
Note that migration logs the process in a log/migrate.log file separate
from the regular Rails log/production.log file.
When you run the migration you'll see something like:
Fixing variants missing unit_value...
Processing variant 12...
Succesfully fixed variant 12
Done!
This helps auditing the changes applied and debug any possible failure
scenarios. You can delete it once all is ok.
