Improve amendment to amendment diff

Description: In an amendment to amendment the difference view doesn't work very well. It is not possible to see deleted content or to recognize differences done in the original amendment content. The common diff-view doesn't work for amendments to amendments.

Reproduction:

1. open a meeting
2. open motions
3. create a motion
4. create an amendment > create an amendment to the amendment and change content of the first amendment.

What should happen:
It would be great to have a difference view for amendment to amendments, which show the differences like in an amendment to a motion. In the current state, the function is quite hard to use, because the it's not clear, what has been changed.

Possible solution: A possible solution could be to display the original amendment content in the follow-up amendment with the format italic. New (added or deleted) parts could be displayed in the same difference style like in first state amendments.

Start by reproducing the issue through a meeting's motions: create a motion, an amendment, then an amendment to that amendment with changed content. Trace the existing amendment-to-motion difference view and determine how the follow-up amendment is rendered. Done means deleted content and changes from the original amendment are visibly represented in the amendment-to-amendment view.