[WIP] Add cmf:media + cmf:file + cmf:image mixin

Also:

- adds unit tests for phpcr File and Image model
- fixes elFinder to add files in a subfolder

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | [TODO]
| License       | MIT
| Doc PR        | n/a

**TODO**
- [ ] check fixed tickets
- [ ] sandbox: remove config as imagine_filters gives an error + add elfinder_thumbnail imagine filter config
- [ ] update changelog