Skills-Search rework: filter_params class rework

[RandomTannenbaum]

To reconstruct the fields of the skill search after a reload and for the view to have access to the results, we have a filter_params class that is provided as a helper by our filter_form controller. In this ticket we want to look over this class and clean it up.

Additionally, add the results directly to this class, so on initialization the results of the search, extracted from the query params, are directly delivered from this class. Then use these results in the view.

ToDo

• [ ] The filter_params class is cleaned up as far as possible
• [ ] The view now directly gets the search results from this class
• [ ] Tests still pass