Add default database values for UserPreference model

In !102897 (merged) we've replaced default_value_for with Rails native implementation. The database allows NULL values and there are a few million records with NULL values. We should backfill those columns with default column values, set a column default with NOT NULL constraints, and corresponding model behaviour.

Implementation plan

• Create migration for setting default value of columns.
• Migration for backfilling of columns with null values to the default value. To be tackled in this issue.
• Model changes to add default database values for UserPreference model. To be tackled in this issue.