Add "parent tracking" to charms/spells

[jakedahn]

We've talked a bit about tracking the parent of each charm/spell as a metadata field on the spell in blobby and maybe storage.

Having this metadata will unlock a few discrete pieces of functionality:

• introduce some sense of "history", allowing us to scrub back and forth over charm iterations
• home page "stacks" where we show the most recent version of a charm, but then you can drilldown and view older versions of the charm
• "spec" history

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tl;dr: let's add a "parent" field to charms/spells so we can go back and forth over iterations/changes

[jakedahn]

Not necessarily part of this immediate issue, but we similarly will want to track "forked_from" to make it possible to traverse the tree of forks/changes

[anotherjesse]

#614 is the start - it doesn't seem to work yet - although I'll rebase an test