Add support for forward proxy auth. Tested with authentik.

- adds new method (`getUserFromProxyAuth) to server/Auth.js to check for
  relevant environment variables and pull the user from the request headers..
 - adds `/forwardauth` route to call new proxy auth method
 - modified /client/login.vue to call `/forwardauth` and authenticate
  the user if returned by forwardauth
 - add a helper method in `server/utils/parseBool.js` to help check for
  boolean values from environment
 - update the readme.md to explain how enable forward auth.

server/utils/parseBool.js

@@ -0,0 +1,8 @@
+const true_values = ["true", "yes", "y", "1", "on"];
+
+module.exports.parseBool = (value) => {
+  if (Object.is(value, null) || value === undefined) {
+    return false;
+  }
+  return true_values.includes(String(value).toLowerCase());
+}