[mobile: iOS/android] setting playercount gets overriden by helper logic

[benjaminxscott]

In a browser on iOS, values for min/max players are reset to defaults when focusing the field and entering a new value e.g. if max_players is 7 then tapping min_players and entering 5 will reset to 7

Workaround is to long press on the field and 'Select All', then enter a value

Likely due to how we reset the element value to max/min based in the validation logic

https://github.com/tobymao/18xx/blob/master/assets/app/view/create_game.rb#L532

Since we have more frontend validation and messaging now, I'd propose:

1. don't reset those values (elm.value)
2. add helper text to the field label e.g. Max Players (7)
3. dogfood to make sure it's usable