Summary:
This adds support for bundling custom widgets with the Grist app, as follows:
* Adds a new `widgets` component to plugins mechanism.
* When a set of widgets is provided in a plugin, the html/js/css assets for those widgets are served on the existing untrusted user content port.
* Any bundled `grist-plugin-api.js` will be served with the Grist app's own version of that file. It is important that bundled widgets not refer to https://docs.getgrist.com for the plugin js, since they must be capable of working offline.
* The logic for configuring that port is updated a bit.
* I removed the CustomAttachedView class in favor of applying settings of bundled custom widgets more directly, without modification on view.
Any Grist installation via docker will need an extra step now, since there is an extra port that needs exposing for full functionality. I did add a `GRIST_TRUST_PLUGINS` option for anyone who really doesn't want to do this, and would prefer to trust the plugins and have them served on the same port.
Actually making use of bundling will be another step. It'll be important to mesh it with our SaaS's use of APP_STATIC_URL for serving most static assets.
Design sketch: https://grist.quip.com/bJlWACWzr2R9/Bundled-custom-widgets
Test Plan: added a test
Reviewers: georgegevoian
Reviewed By: georgegevoian
Differential Revision: https://phab.getgrist.com/D4069
Summary: This moves enough server material into core to run a home server. The data engine is not yet incorporated (though in manual testing it works when ported).
Test Plan: existing tests pass
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D2552
