Add support for nested metrics

[SUPERCILEX]

The big issue with metrics right now is that you can't display them conditionally. For example, in this list we would like to say "If they have a bulldozer, is it external or internal, can it move bunnies, and can it move buckets?" Otherwise, those options don't matter.

Bulldozer? (list)
  None
  Internal
  External
  Bunnies
  Buckets

Instead, we have to ask each question separately assuming they have a bulldozer which makes things super messy. An ideal structure would look like this:

Bulldozer (checkbox, only shows items below if checked)
  Type (selector)
    Unknown
    Internal
    External
  Can bulldoze bunnies (checkbox)
  Can bulldoze buckets (checkbox)

Code wise, I'm not sure if this is possible while maintaining decent performance. We'd have to store each metric inside that one root metric which is a terrible idea. Also, we'd have to figure how to sort and update items within that root metric asynchronously plus get a nested vertical RecyclerView working. For exporting, we'd have to flatten those nested metrics out.