Update ImagineBlock to use CmfMedia image

See https://github.com/symfony-cmf/cmf-sandbox/pull/189 for related CmfMedia PR's.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | na
| License       | MIT
| Doc PR        | https://github.com/symfony-cmf/symfony-cmf-docs/pull/191

**TODO to merge**
- [x] Needs to be rebased on master and updated after recent mappings update and cleanup
- [ ] Add web test case for image block
- [x] ``Doctrine\Phpcr\ImagineBlock``
  - use ``ImageInterface`` for checks instead of Image document?
  - use configuration parameter for Image document class?
- [x] imagine block template
  - check how to pass image object to imagine_filter
  - add to changelog if changed
  - document if using ``image.id``