HF20 account creation operations

[roadscape]

Changes

• adds claim_account and create_claimed_account
• removes challenge_authority and prove_authority
  • no longer in steem_operations.hpp
  • op indices seem to have been repurposed

Unresolved

• what to do with createAccount(options: CreateAccountOptions, key: PrivateKey) in helpers/broadcast.ts?
  • is it used anywhere?
  • to upgrade, would need two paths
    1. create account with fee, then delegate SP
    2. create claimed account, then delegate SP

[bnchdrff]

i think we should increase the major version by 1 and remove createAccount - it's a significant enough change right?

[jnordberg]

LGTM 👍

[roadscape]

Replaced with #4