Commit 63e50786 made stop words go through when tokenizing FTS5 query
search terms, in order to still provide matches for incompletely typed
search terms that happen to match a stop word.

This however brought the side effect that searching for a stop word in
combination with other terms renders the latter ineffective. As the stop
word has no tokens in the FTS5 table to match with, the whole query brings
no results.

Since that commit, SQLite fixed FTS5_TOKENIZE_PREFIX to work as advertised,
so limit the bypass to prefix queries (e.g. "onto*"), since it only makes
sense there. Also, invert the way we look for stop words (i.e. always lookup
those in search terms as per config, and do the bypass once we know we deal
with a stop word) for the sake of readability.

https://bugzilla.gnome.org/show_bug.cgi?id=787452