Let dev-support _support_ SSF based extensions

[elia]

Desired Behavior

Let dev-support help with extensions that work with solidus_starter_frontend.

• Switch to a lightweight release process (use bundler gem tasks and GitHub releases)
• Setup scripts for generating, updating and synchronizing the sandbox and dummy app
• Generate an updated solidus extension skeleton
• Drop all the currently unused features

These updates should mostly come from real-world usage, and the most advanced extension available that is supporting SSF is solidus_stripe.