UX improvements

[xSke]

• System/member edit command rework (see Refactor command structure #6)
• Better status information and prompting
  • Member name conflict prompt (see Add a confirmation dialog if there's a member name conflict #10)
• Better errors
  • Various hints
    • Hint "quoting names" on unresolved member name
    • More specific help pages on unknown subcommand (instead of just ignoring)
  • Avatar size/format check (see Make the avatar updating command check file size (and possibly resize) #5)

(do comment with more suggestions!)