Add FieldLoader support for Nullable<T>.
Allow maps to override options. Closes #3646.

Also set sensible defaults for most of the maps and mini games.
Reorganize map options panel.
Add "Build off Ally ConYards" option. Fixes #2464.
Remove dead InitialCash setting from PlayerReference.
Add starting cash option.
Disable crates option for missions and mini games.
Tweak layout again.

C&C doesn't offer fragile alliances.
Merge pull request #3687 from pchote/mapoptions

Lobby map option improvements.