Investigate CircleCI Contexts for sharing envvars between projects

[hoverduck]

With all of the different 'wrapper repos' used to run the different sets of e2e tests (canary / visdiff / i18n / etc), we have separate copies of the NODE_CONFIG variable unique to each one. Some keys like the Slack webhooks need to be different in each of them, but some are the same across all. CircleCI recently added a feature called Contexts that lets you share config variables across projects, and I'd like to explore using that to simplify our config.

Ref: https://circleci.com/docs/2.0/contexts/

Without actually digging into it yet, here are the problems I can think of to start:

1. The Org Settings page is only accessible to organization admins, so whoever is working on this would need to be added to that list.
2. The documentation linked above is only for CircleCI 2.0, and all of our wrapper repos are still using 1.0. I'm not sure if that means we need to upgrade them (which shouldn't be a big deal) or if they'll still inherit the envvars
3. The config npm package only references a single $NODE_CONFIG variable...we'd need a way to specify a secondary envvar to override specific elements of the parent, like Slack hooks