Implements core NIP-26 delegated event functionality. Events can
include a `delegation` tag that provides a signature and restrictions
on which events can be delegated.
Notable points on the implementation so far:
* Schema has been upgraded to include an index and new column.
* Basic rune parsing/evaluation to implement the example event in the
NIP, but no more.
* No special logic for deletion.
* No migration logic for determining delegated authors for
already-stored events.