implement liquid democracy delegations

[brenzi]

Allow voters to delegate their vote to someone else.

Delegations shall be

• quasi static (valid until they are changed)
• scoped
  • Any
  • AnyLocal(CommunityIdentifier)
  • Specified(Vec<ProposalAction>)
• recursive (up to N layers deep)
• lazy (it is up to the delegate to cast a vote for all delegators, moving complexity off-chain)

note

• consider using vouches pallet to register delegations