LPS-44862  "Link to Page" field's getUrl method doesn't work properly

Hi Máté,

you've already reviewed some of the changes from this pr, however Brian didn't merge it as he requested tests, so I added some.

Could you please review the changes related to ExportImportHelperUtilTest at least?

I've separated the tests when the layouts belong to a user, because it makes no sense to set up staging between two groups and export/import contents from a third (user) group.

In this case I've skipped the tests related to the import process because of two reasons:
(1) I think it would add unnecessary complexity if I set up the live and the staging groups as well.
(2) The testImportLinksToLayouts() method needs some update, as currently the layoutId and groupId replacement doesn't tested at all.

Thanks,
Tamás