Sandbox OpenID Connect identity provider for local development

[lpsinger]

Implement a sandbox OpenID Connect identity provider for local development so that parts of the web site that require login function without connecting to the real Amazon Cognito identity provider.

1. Add node-oidc-provider as a dev dependency.
2. Create an Architect plugin in the src/plugins directory and add it to the @plugins section of app.arc.
3. In the plugin, add a sandbox.start hook that initializes a node-oidc-provider Provider and runs it on an open port. It can have a single, hardcoded user account.
4. In the plugin, add a set.env hook which populates an env variable with the OIDC .well-known/openid-configuration metadata: from the sandbox identity provider if running in sandbox mode, or from the Cognito identity provider if running in production mode.
5. Update app/routes/__auth to configure node-openid-client using the OIDC metadata returned by the plugin.

This will close #91.

[lpsinger]

Closed by #449.