List location groups

A location group is a set of locations listed together as one brand. On directories that support it (Localtunity today, and the regional sites that copy it), the group is one business named after the group, and each location appears under it with its own address. Every other directory keeps listing each location on its own. Use it for a chain, most often when its locations share a phone number, because those directories accept only one listing per number. Returns each group the key can see, plus `sites`: the directories that list groups together. With a key limited to some clients, counts and client ids cover only those clients; `hiddenLocationCount` counts the rest.