Search should keep category filter

[nils-van-zuijlen]

Currently

1. when on a search result page, clicking on a category clears the search.
2. when on a category page, filling in a search clears the category filter.
3. when on a category page, adding the search query to the URL does the search in the category.

It would be nice if the actions of 1. and 2. would result in a navigation to the page shown in 3.

For example, when on https://juliapackages.com/packages?search=serial clicking on "File IO" in the sidebar leads to https://juliapackages.com/c/file-io but should lead to https://juliapackages.com/c/file-io?search=serial