chore: Move remaining auth methods to plugins (#4900)

* Move Google, Email, and Azure to plugins

* Move OIDC provider, remove old loading code

* Move AuthLogo to plugin

* AuthLogo -> PluginIcon

* Lazy load plugin settings

docs/AUTHENTICATION_PROVIDERS.md

@@ -0,0 +1,14 @@
+# Authentication Providers
+
+A new auth provider can be added with the addition of a plugin with a koa router
+as the default export in /server/auth/[provider].ts and (optionally) a matching
+logo in `/client/Icon.tsx` that will appear on the sign-in 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