fix(signup): signup auth (#15628)

* fix(models.user): Colocate all user methods

Moved user methods/extensions into one file. Tracked down `next method
called more than once` error and setting headers after their sent. Let
regular error handler handle api errors as well.

* feat(server.auth): Disable github account creation

We are no longer allowing account creation through github

* refactor(Auth): Move user identity link into models dir

* feat(Disable link account login): This removes the ability to use a linked account t

* feat(errorhandlers): Add opbeat, filter out handled error

server/utils/create-handled-error.js

@@ -0,0 +1,18 @@
+const _handledError = Symbol('handledError');
+
+export function isHandledError(err) {
+  return !!err[_handledError];
+}
+
+export function unwrapHandledError(err) {
+  return err[_handledError] || {};
+}
+
+export function wrapHandledError(err, {
+  type,
+  message,
+  redirectTo
+}) {
+  err[_handledError] = { type, message, redirectTo };
+  return err;
+}