Migrate custom licenses to the new table.

We want to migrate the custom licenses to the new table created in MR !151445 (merged)

Implementation plan

1. Add a data migration to fill the new table with the license name and project_id

# frozen_string_literal: true

class CopyCustomLicenses < Gitlab::Database::Migration[2.2]
  milestone '16.10'

  def up
    sql = <<-SQL
      INSERT INTO custom_software_licenses (name, project_id)
      SELECT
          name,
          project_id
      FROM
          software_licenses
          JOIN software_license_policies ON (software_licenses.id = software_license_id)
      WHERE
          spdx_identifier IS NULL
      GROUP BY
          name,
          project_id;
    SQL

    execute(sql)
  end

  def down
    # no-op
  end
end

We are expecting duplicated licenses among projects.

License	project
custom	432
custom	165
custom	137

2. Remove the method unclassified_licenses_for on SoftwareLicense

2.1 Remove the test cases related to this method from ee/spec/models/software_license_spec.rb

3. Update software licenses with custom licenses.

UPDATE software_license_policies
          SET software_license_id = NULL,
          custom_software_license_id = custom_software_licenses.id
        FROM
            custom_software_licenses
            JOIN software_licenses ON custom_software_licenses.name = software_licenses.name
        WHERE
            software_licenses.spdx_identifier IS NULL
            AND custom_software_licenses.project_id = software_license_policies.project_id
            AND software_licenses.id = software_license_policies.software_license_id