Change rules to allow invalid attackers.

[BlakeMScurr]

Alternative rule simplification to https://github.com/TruncateGame/Truncate/pull/247. This is better because keeping "strongest attack vs weakest defender" is arguably intuitive because of attackers' initiative in actual battles, and because this makes it harder to shoot oneself in the foot when attacking (rather than easier) which is a major pain point in onboarding.

The update tutorial is phrased as though it is thrust on the user. But I still think we should make it an optional pop up, as many (most? all??) of our users dislike reading through tutorials. Moreover, many of the people who see this update won't have digested the original rules anyway, meaning being forced to read this will be extra taxing and possibly confusing.

[bglw]

Agreed that we should make a popup. Should we include that in this PR or merge this to outpost and resolve popup later?