Amendments: change recommendation diff doesn't work

Description: Recommendations with crossed out lines done in amendments are not displayed. The line will just disappear without a diff-hint. This isn't optimal. It is not easy to recognize, what content is crossed out.

Original amendment:

Crossed out sentence:

Reproduction:

1. open a meeting
2. open motions
3. create a motion
4. add an amendment to the motion
5. add a change recommendation to the amendment content by deleting a word or more content.

What should happen:
The diff should display in the amendment the crossed out line done by the change recommendation. With this change users could easily spot diffs. Also the diff would be visible in the projection.

Note: The issue is linked to #2503