Improved Handling of Soldier Ranks over 7

[Tzarnal]

Ever since Long War 2 the modding community standard for custom classes has shifted to 8 ranks with 3 abilities per rank. While more than 2 abilities per rank are fairly easily implemented more than 7 ranks is not.

Ideally 7 rank soldiers and soldiers with higher amount of ranks should peacefully coexist. But the moment you implement an experience table rank entry for 8 or higher things go wonky. And the solutions tend to live in XComGameState_Unit.

I'm not aware enough of the codebase as a whole go give comprehensive suggestions for what to do here but I've got some starting points.

First and most obvious:

2:57 PM]  robojumper: Right, `GET_MAX_RANK is global for everyone
[2:58 PM] robojumper: X2SoldierClassTemplate has a function GetMaxConfiguredRank()
[2:58 PM] robojumper: We should probably replace calls to `GET_MAX_RANK with those
[2:59 PM] robojumper: Care to open an issue in the HL?

Second:

When calling a XComGameState_Unit.GetRankAbilities(int Rank) for an unavailable rank it returns the squaddy rank Abilities. I suggest it instead return EmptyRankAbilities.