Make FeatureMediaBlockService not extend from MediaBlockService

[jordisala1991]

In the process, getStylesheet is removed since it is not used on
BlockBundle 4.0, and load is refactor to ensure we can load the
correct entity with its given id (not necessarily a integer).

Subject

I am targeting this branch, because this breaks BC.

Changelog

### Changed
- Changed `FeatureMediaBlockService` to not extend `MediaBlockService`.
- Changed `MediaBlockService` to be final.

[wbloszyk]

This property make some issue.
When we set db_driver: 'no_driver' then sonata.media.admin.media does not exists and throw exception.
We should:
Allow create admin service with manager_type: none (or manager_type: no_driver).
Or make mediaAdmin optional - then also we should not allow generate edit form without mediaAdmin.
Also if we do not have admin, then we should throw exception while generate edit form (is used by EditableBlockService). So can you change it to optional?

[wbloszyk]

Or we should find way to replace them.

[jordisala1991]

This is working exactly the same as the MediaBlockService.
The no_driver option is not suposed to be used by the users, it is just a temporary option.

If you want to improve that part. Please create another issue explaining why is that important and/or open a Pull request showing some code to implement that part.