Base Distribution

[FEATURE] Refactor, fix and improve image handling

This change implements reworked, cleaned up and fixed image handling.

NOTE: If you add an image content element and directly want to modify it,
the "apply" button does not activate. This is a bug in adding of nodes,
as described in #40860. This will be fixed lateron. As a workaround,
you should reload the backend after adding a new image content element.

It contains the following detail-improvements:

• Uploading images works correctly
• Cropping images works
• Setting width and height works as expected
• saving works
• loading existing images works
• We now consistently use "w" and "h" for width and height, in
  sync with jCrop
• updated jCrop
• various detail bugfixes
• code cleanup and restructuring
• When selecting an already-existing image, a loading indicator
  is shown while image metadata is fetched.
• The server side ContentController is cleaned up, containing only
  the methods which are still used.

Resolves: #40691
Change-Id: Id640f0ce45fc4145336c66fe18823299f866b46b