package and distribute as slack app

[lucacioria]

requirements

• slack app with command and bot permissions
  • use slash commands as now, but reply using the bot
• create page to install app at /add-to-slack
• create dev test environment
  • slack team pledge-test
  • slack app pledge-test
  • command pledge-test
  • use ngrok with reserved subdomain for local testing

specs

{optional: describe technical specs to implement this feature, if not obvious}

misc

{optional: other useful info}