Improve local development for external services

[mastercactapus]

What problem would you like to solve? Please describe:

• Hard to know what local tools are available
• Onboarding currently requires creating accounts and credentials for external services
• Getting publicly-routable URLs to local developer machines isn't always feasible

Describe the solution you'd like:

• Re-use existing mock servers for dev, in addition to testing
• Link to existing tools (e.g., MailHog) where developers can find it

Describe alternatives you've considered:
Currently, developers need to have access to all services they are developing against, even for established features that have mock servers for testing.

Additional context:

[mastercactapus]

Wireframe for Twilio UI concept

[mastercactapus]

Will open new issues for:

• mocktwilio UI
• mockslack UI
• Admin/Config history
• Dev tool for sending things like Grafana alerts