initial prototype of a interactive search input using select2.js

this has its warts, e.g. the filters are kept in the `q` query parameter instead of correctly using the `topic` query parameter multiple times per selected filter. I can think of two ways out:

- rewrite browser history and add `topic` parameter based on selected filters, extend `initSelection` function in `search.js` to take those parameters into account, too
- store selected topic filters in hidden form fields after being selected in select2 and let the form serialization takes its place natively by the browser on submit

I don’t quite know how to proceed with this, tbh. I lack the experience with implementing such a UI widget using JavaScript.

If the integration with select2.js is too costly or otherwise not a good idea, I could also see a simpler `<select>` based implementation of a filter dropdown next to the search input.