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.
This looks like wrapper function that does 2 things: 
- set poi link (either wiki article or external
- set poi photo based on wiki article

could we create separate functions for both actions and call in here based on different conditions?