Big refactoring of document roles

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | 
| License       | MIT
| Doc PR        | none yet

- This PR changes the role of [Menu|MenuNode] to the standard
  implementation - "the document that integrates with other CMF
  components" whilst MenuNodeBase simply makes KnpMenu persistable and
  adds nothing more.

- The Multilang specific classes have all gone.

- There are now only 2 admin classes - both for the standard
  implementation