Invoice Canister Cleanup Tasks

[krpeacock]

• [x] Ensure that person without viewing permission cannot view an already verified invoice
  • invoice.mo line 283
• [x] Gitignore src/declarations
• [x] replace dfx stop with `#!/usr/bin/env bash; set -e; dfx stop
• [ ] Add access control for creating new invoices
• [x] Check account for transfer fee during transfer
  • ICPLedger.mo line 179
• [x] pull magic numbers into constants
  • main.mo line 139
• [ ] Refactor permission checks to a method