Added support for many-to-many inline editing in sonata_type_collection

Usage of this feature is exactly same as in one-to-many relation.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | sonata-project/SonataAdminBundle#106, #234
| License       | MIT
| Doc PR        | -