feat: authenticationProviders API endpoints (#1962)

server/auth/providers/README.md

@@ -0,0 +1,13 @@
+# Authentication Providers
+
+A new auth provider can be added with the addition of a single file in this
+folder, and (optionally) a matching logo in `/app/components/AuthLogo/index.js`
+that will appear on the signin button.
+
+Auth providers generally use [Passport](http://www.passportjs.org/) strategies,
+although they can use any custom logic if needed. See the `google` auth provider for the cleanest example of what is required – some rules:
+
+- The strategy name _must_ be lowercase
+- The strategy _must_ call the `accountProvisioner` command in the verify callback
+- The auth file _must_ export a `config` object with `name` and `enabled` keys
+- The auth file _must_ have a default export with a koa-router