Fix EZP-23513: embed rendering (RichText)

This PR resolves https://jira.ez.no/browse/EZP-23513

Part 2 of 2, handling RichText field type.
Part 1 of 2 for XmlText can be found here: https://github.com/ezsystems/ezpublish-kernel/pull/1087

#### Followups:
* todo