Remove hat module

It's not been updated for 6 years!

src/users.js

@@ -37,7 +37,7 @@ var assert = require('assert'),
     groupdb = require('./groupdb.js'),
     groups = require('./groups.js'),
     GroupsError = groups.GroupsError,
-    hat = require('hat'),
+    hat = require('./hat.js'),
     mailer = require('./mailer.js'),
     qrcode = require('qrcode'),
     safe = require('safetydance'),