Fix EZP-22969: Updating Content with RichText adds entry to ezurl, but not to ezurl_object_link

This PR resolves issue https://jira.ez.no/browse/EZP-22969

When creating or updating RichText field with content containing an external link, entry will be created in the ezurl table, but not in the ezurl_object_link table.
This causes the urls to later be incorrectly considered "orphaned" and removed (see Url FieldType).

Corresponding fix for XmlText: https://github.com/ezsystems/ezpublish-kernel/pull/885

While testing this discovered another issue: https://jira.ez.no/browse/EZP-22999

### TODOs
- [x] XmlText and RichText: always link the URL (not only when it is being created)
- [x] XmlText and RichText: if the same URL appears multiple times store/link it only once