Remove the implicit top level filters

Currently, top_level=true implies either folder=true or type=corpus.top_level_type. This mean a top-level non-folder element is simply invisible, or it is impossible to get top-level elements of another type. The filter bar allows us to put a lot of filters, so we could just have the frontend default to top_level=true + folder=yes or top_level=true + type=top_level_type but allow the user to remove those filters if they want to, and do just any query they want.

This requires #830 (closed) to make the frontend able to access the top-level type.