Search in top bar should start a new search

[tuukka]

What

I'd expect a search in the top bar to always start a new search instead of refining any existing search.

Steps to reproduce the behavior:

1. Go to https://search.openfoodfacts.net/
2. Type in "categories:teas" and press enter.
3. Decide you want to search something else, replace the previous text with "categories:pastas" and press enter.
4. Observe the error: You don't get pastas as a result, mainly because it's trying to search for both teas and pastas at the same time (but also because this currently causes a 500 error at the backend.)

Expected behavior

I would expect a search in the top bar to always start a new search (and reset any existing search).

To refine an existing search, I would expect there to be another search field with an indication such as "Refine your search". How search refinement should work (logical "AND", "OR" etc.) is a topic for another issue.