Multiple negated filters on issuable search does not work as expected

Adding multiple negated filters for issuables, like in this search only includes one of the negations the actual query being performed.

Looking into the queries being performed, only 1 author_username is being passed.

The search results included results that I would expect to be filtered out:

If we do want to support multiple filters like this, we should put a limit on how many we allow being passed into the query. If we don't want to support this, we should prevent them from being selected in the frontend.