[Core] Improves file handling.

* Adds saveCopy method to FileMapper
* Fixes and Improves FileEntityBuilder, FileRepository and FileEntity:
  Now FileEntities can return a php resource for the file content.
[Auth] Adds hidden form field for the profile image id.
[Applications] Adds contact images to the application form.

If no image is uploaded, but a user is logged in, the profile image will be copied
to the application.
Merge branch 'feature/Applications-contact_image' into develop