Add metrics for nested packs

[alexevanczuk]

This adds metrics for nested packs!

A couple of design decisions, framed around language:

• every pack is in a pack group
• a pack with no children is not a parent -- it is a standalone pack
• a pack is only a parent if it's in a pack group with more than one member and its name is the name of the pack group
• a pack is only a child if it's in a pack group with more than one member and it is not the parent

What this means:

• pack group counts will go DOWN over time
• the count of parents and children will go UP over time
• Our DD dashboards should look at percent of packs that are parents or children (i.e. (parent packs + child packs)/total_packs).