SeoMetadata as mapping instead of listener work to assoc

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | #149 
| License       | MIT
| Doc PR        | need to update https://github.com/symfony-cmf/symfony-cmf-docs/pull/442

The following tasks need to be done:

* [x] persisistence for phpcr-odm
* [x] persistence for orm
* [x] just key, values instead of `Extra` object
* [x] form type for those key-value-pairs
* [x] configuration @WouterJ ?
* [x] clearing CS