Fix inability to set visibility_level on project via API

Consider the scenario:

1. The default visibility level is set to internal
2. A user attempts to create a private project within a private group

Previously this would always fail because default_value_for would overwrite the private visibility setting, no matter what visibility_level were specified. This was happening because default_value_for was confused by the default value of 0 specified by the database schema.

default_value_for attempts to assign the default value in the block by checking whether the attribute has changed. The problem is that since the default value by the database was 0, and the user requested 0, this appeared as though no changes were made. As a result, default_value_for would always overwrite the user's preference.

To fix this, we remove the use of default_value_for and only set the visibility level to the default application setting when no preference has been given at creation time.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/63158