WIP: Create a plugin interface for wp-admin from Calypso

[ryelle]

At the JPOP Design+Luna meetup in Amsterdam, the idea of building a plugin (for wp-admin) out of Calypso was proposed (again). This PR is the product of a few days exploration into the idea.

What's been done here:

• A new project config has been added, called jetpack here but could be made more generic or copied for wp super cache or other plugins
• The main boot file has been forked here so that API-dependent code could be pulled out, otherwise nothing would load outside of whitelisted URLs
• A custom Layout was added, but this might be unnecessary
• 2 new sections were added that only exist in the Jetpack project, a "writing" and "discussion" UI (my head is still in the Jetpack settings project 😄 ), these link to each other.
• The page options are updated for this project for better compat with wp-admin paths (react-router might be a better approach for this?)
• A shell script moves the built javascript files into a directory defined by JETPACK_DIR, this was just for easier local testing, and would change once we decide how this process should work

A screen recording of the demo plugin: https://cloudup.com/cKou6HLSNpi

To test this yourself:

1. Download the demo plugin from this gist and add it to your WP plugins directory
2. Update JETPACK_DIR in bin/copy-files.sh to point to the folder you've just created for the plugin (or anywhere on your computer where you can then upload the files to your wp install, if it's not local)
3. Run npm run build-jetpack && ./bin/copy-files.sh — This should create a folder public in the plugin, with 3 JS files and a style.css
4. Load wp-admin, activate your new plugin called "Jetpack UI Test"
5. Now you should have a item in the admin menu called "Jetpack UI", and clicking will bring you to the pages generated from calypso
6. You can click back and forth between a "writing" and "discussion" page using the buttons 🎉

[matticbot]

Test live: https://calypso.live/?branch=try/jetpack-build-process

[apeatling]

This is a great exploration, although it looks like it has come to an end. I am going to close this as I don't see a path to merging. We can still use it for reference in the future, and re-open if it is restarted.