Rename local variable from "active" to "grid_active"

Rename local variable from "active" to "grid_active"

There is an "active" property up and down our component stack, (at the
board level, whether it's the current players turn; further at the
mini-grid level, whether it's a legal mini-grid; further still at the
square level, whether the square is unoccupied). It's clear enough to
uses a props name of "active" at each level.

But when using a local variable to compute the child's "active" prop
while in a current component with its own "active" prop, this can get
confusing.

So, at the board level, use a local variable of "grid_active" to
compute the "active" prop for the child grid.