Change UploadFileHelper to an interface and Doctrine implementation

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #66
| License       | MIT
| Doc PR        | https://github.com/symfony-cmf/symfony-cmf-docs/pull/276

To be merged after this PR:
- https://github.com/symfony-cmf/CreateBundle/pull/81