more detailed update message during catalog import

[MyPyDavid]

Rationale / Begründung

An editor wants to check how the new and old version of the catalog differ.

When importing a new version of a catalog, for every existing entry a message "This element will be updated." is shown. It would be helpful if exact changes are listed during the import as well.

Affected

Managers

Minimal functionality

• a more detailed information is given what is changed during the update (e.g. similar to git diff)

References / Verweise

• Nearly similar to https://github.com/rdmorganiser/rdmo/issues/272
• https://github.com/rdmorganiser/rdmo/issues/653