MWEB-940 Add possibility to save external links for POIs on maps

When we added autocomplete to associated articles we lost the ability to link to external URLs, and we've gotten feedback from the beta tester community members that they miss that feature.

In the code changes below we've changed placeholder so it informs a user can add internal article or external link to POI's data. We've changed error messages and validation and added unit tests.
could we move this to separate method and create unit test for it?