We were using very similar code for proposals, debates and investments,
so we might as well share the code between them.
Note we're using the `proposals.index.search_results` key even for
debates and investments. This will still work because the translations
shared the same text, but IMHO we should rename the key to something
like `shared.search_results_summary`. We aren't doing so because we'd
lose all the existing translations.
