Remove gitlab_custom_project_template from import_sources column

Problem

Custom project template export was accidentally added to import sources, which allowed admins to enable it as an import source.

As a result, import_sources column in application_settings table can include gitlab_custom_project_template value.

Proposal

1. Create a migration to remove gitlab_custom_project_template values from import_sources in the application_settings table
2. Adjust validation to prevent gitlab_custom_project_template from being added as an import source