Update period end calculation to permission end on overflow

[ilikesymmetry]

This does not change any functionality, but maintains our data clarity when the final period exceeds the permission's end. The largest a PermissionSpend.end will ever be is equivalent to SpendPermission.end which follows the intended use case. Prior, we were letting PermissionSpend.end > SpendPermission.end theoretically which was misleading because other logic enforces SpendPermission.end as the absolute end of using a permission, regardless of period spend ranges.

[amiecorso]

nice, this actually makes the logic in getPeriodSpend more intuitive to read too