fix: content vs element

[marrouchi]

Motivation

This PR fixes things about lists and carousel. We have now :

1. Content (that is the object as stored in DB - with dynamicFields)
2. ContentElement which is a flattened form of the content. It would be used in list / carousel ("elements" attribute)

This should solve the confusion around both, both in channel and plugins.

Type of change:

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[IkbelTalebHssan]

Fix tested and approved 🚀 ✔️