Add daily challenge mode

[miridius]

• [x] By default when coming to the app, everyone should see the current day's country
• [x] Switchover should happen at UTC midnight rather than local time like worldle
• [x] Browser local storage and/or cookies (firestore?) should be used to make sure they only attempt it once.
• [ ] #36
• [x] We should screen upcoming days' countries ahead of time to make sure they don't suck (or use a hand-curated list)
• [ ] #37