#56 Storing thread notifications in the database

Currently the database only stores hard facts about each thread. It doesn't directly store booleans that basically amount to "should the user see this thread". These are evaluated when `list_thread` is called. It might simplify the code if I just add some boolean columns to the db, ~~maybe they can even be auto-calculated~~ (no computed columns in SQLite apparently). The cost would be a more stateful program with more side-effects. The reason I'm concerned about this is the line `if (mark != ' ') or (not interesting_only):` in `list_threads` - it seems very hacky but I can't improve on it either.