Keep swisssearch parameter in URL if possible

This PR adds a more sensible handling of the swisssearch parameter removal. Before, it was removed without consideration on appliation load. Now, it's removed when

1. when a result in the dropdown list is selected (exception: see 3)
1. when query is emptied with the cross in the searchbox
1. when swisssearch parameter results in a single result AND the map is moved/zoomed

In all other cases, the parameter is kept.

Note: in theory, we could _add_ a swissearch paramter when the user enters a query...and then apply the same logic as above. If this is desired, I can add it to this PR.