Image and File field enhancements (+language file strings)

Currently:
An image/file fields will be displayed with upload or remove button even if it has the "editable" option set to false.

With this PR:
If an image/file field is not editalble:
The remove and the upload button is not visible.
However, uploaded images and files are still visible.
If no image or file has been uploaded, a localization of 'No image uploaded' or 'No file uploaded' will be displayed according to the language files.

Maybe closes: #408 (datetime works for me)