Include information about visited URL in non-short URL visits

[acelaya]

Summary

The Visit model does not include information about what short URL has been visited. The reason is that originally you could only fetch visits for one short URL, so it was implicit.

With time, Shlink has evolved to support fetching visits for a tag, domain or visits in general.

In those cases it is not clear what was the visited URL that generated that visit. It would be useful to include that as part of the exposed model.