Fix EZP-21580: Creating content with URLs does not store url links correctly

This PR resolves issues [EZP-21580](https://jira.ez.no/browse/EZP-21580) and [EZP-21601](https://jira.ez.no/browse/EZP-21601)

For Url field type external data was not properly stored, created URL was not linked with content Field.
Also deleting was not implemented.

Tested manually.

Additional: various cosmetic fixes.