Make boardSelector memoizable

[inooid]

As described in the redux recipe about using selectors we are using a selector in two different containers. This will make the selector not memoizable, so we might want to rewrite that into a function as described here: https://github.com/reactjs/redux/blob/master/docs/recipes/ComputingDerivedData.md#containersvisibletodolistjs-2

File: src/redux/modules/board.js