[WIP] Elfinder

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #28
| License       | MIT
| Doc PR        | https://github.com/symfony-cmf/symfony-cmf-docs/pull/251

**TODO to merge**
- [ ] code review / cleanup
- [ ] check root creating (see PHPCRDriver:: _stat)
- [ ] check rename error (DocumentManager::move results in an error)
- [x] add installation documentation
- [x] recheck image preview with imagine thumbnail - elFinder uses the same url for full screen display
- [ ] check if it is possible to return a placeholder for the url and use a filter or something when rendering content (use a dataFilter rule for ckeditor)

//cc @sjopet