Multiplayer & opportunity fixes, minor design tweaks

- Various fixes to reduce multiplayer desyncs and 'unable to load game' errors.
- Games that show 'Timers Expired' are now rejoinable more often.
- Fixed a rare blocker when an opportunity triggered during the end turn sequence.
- Fixed a rare blocker where an opportunity would trigger multiple times.
- Sandtimer and eraser buttons now display above knowledge/wealth panel so you can always reset turn.
- Resetting turn enabled when choosing an opportunity reward.
- Some confusing opportunities have been clarified.
- Germans have been renamed Celts in certain languages.
- Fixed some opportunities being offered when they were useless.
- Fixed 0 crown wonders showing on victory panel in multiplayer.
- Difficulty ratings adjusted: Song to Easy; Chu, Zhou and Mycenaeans on Mediterranean to Medium.
- Deep sea locations tweaked ahead of us adding a visual indicator of where deep sea is.