Menu referrers - WIP do not merge!

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony-cmf/ContentBundle/issues/39 (needs more work to fix)
| License       | MIT
| Doc PR        | TODO

adding an admin extension so we can move out the menu specific stuff from ContentBundle.