Outstanding work for member management.

proxy

• list members for org
• add member endpoint
• remove member endpoint (later feature)
• refactor user org list to filter by handle instead of the author key

proxy <> ui

• use registration endpoint when submitting the transaction in the modal
• validation: do not allow registering an existing member

ui

• add member flow
  • first member flow iteration ( feat(ui): visuals for add member to org flow #334 )
  • show avatar/imageURL of user in text input
• member list
  • first member list iteration ( feat(ui): org members list #402 )
  • navigate to user page
  • use actual member list
• ensure correct handling of member txs in transaction center
• thorough testing: additional users currently can only be created in the testing environment

Part of #277