codeintel: Implement protected configuration policies

Created by: efritz

Please review by commit. This PR adds a protected field to the lsif_configuration_policies table and exposes it to the UI. This field is meant to be enabled only for "system policies" which shouldn't be easily deleted or modified (a base age for all branches and tags). This will prevent site-admins from accidentally deleting all of their data when they fat-finger a data retention policy.