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/server.js

@@ -1,7 +1,13 @@
 require('dotenv').load();
-var pmx = require('pmx');
 
-pmx.init();
+if (process.env.OPBEAT_ID) {
+  console.log('loading opbeat');
+  require('opbeat').start({
+    appId: process.env.OPBEAT_ID,
+    organizationId: process.env.OPBEAT_ORG_ID,
+    secretToken: process.env.OPBEAT_SECRET
+  });
+}
 
 var _ = require('lodash'),
     Rx = require('rx'),