update cog init to include a GitHub Actions workflow file

[zeke]

This PR adds a "Push to Replicate" GitHub Actions workflow file to the cog init template, so users will have a smoother path to continuous delivery.

The workflow is configured to start users off with a manual "repository dispatch" so pushes are explicit and intentional. It also has some commented-out settings to support a more continuous workflow once they've kicked the tires, plus some pointers for common issues like running out of disk space.

$ ~/go/bin/cog init     

Setting up the current directory for use with Cog...

✅ Created /Users/z/Desktop/cog-init-test/cog.yaml
✅ Created /Users/z/Desktop/cog-init-test/predict.py
✅ Created /Users/z/Desktop/cog-init-test/.dockerignore
✅ Created /Users/z/Desktop/cog-init-test/.github/workflows/push.yaml

Done! For next steps, check out the docs at https://cog.run/getting-started

[yorickvP]

The test is failing because of formatting.