Ensure that selectors for getting the "My groups", "Currently viewing
groups" and "Featured groups" lists only recompute when the groups state
changes. Previously they would have recomputed when _any_ part of the state changed.

More generally, we should be using `createSelector` rather than
`memoize` whenever we have a place in the app that needs to extract a subset
of data from some larger structure and derive some data from that.