fixes #8477 - usability improvements to template locking/defaults

- In practice, locking templates without the ability to unlock them doesn't seem to work (see the foreman_discovery issue).
- Provide a warning for templates that are shipped by default by plugins
- Exposes the 'default' option to users so they can mark templates to be automatically seeded or not
- Mark foreman templates as default on new installs

If this or something like this is taken, I can remove the code from Katello seeds that touches Foreman templates.