- Add check_constraints_for(table, column, schema: nil)
  private method to Gitlab::Database::MigrationHelpers.
  Returns all the check constraints defined for a column.
- Add the copy_check_constraints(table, old, new, schema: nil)
  migration helper.
  It copies all co straints defined in column old to column new.
- Update the create_column_from migration helper to
  also copy all existing check constraints to the new column.
- Update the specs for all helpers that use create_column_from
  and add new specs for copy_check_constraints