refactor: question route

[kaljarv]

The page displaying a single question needs a rework. Note, though, that slotless page templates (#375) and app state management (#536) will have effects on this.

• Split this into parts so that it can be more easily used in the Candidate App
• Especially separate the category intro and the question display
• Refactor the update logic which now happens inside updateQuestion, possibly to a different stream that filters and orders the available questions and another one dependent on that and the questionId route param that selects the question to display.