Add initial support for multiple companies

You can find screenshots and other files
related to the feature on Dropbox here:
https://www.dropbox.com/sh/wugnzqepwgzwlnh/AAAwNWA5hIKrP0y2VTu-LMHHa?dl=0

https://github.com/snipe/snipe-it/issues/23

app/models/User.php

@@ -12,6 +12,11 @@ class User extends SentryUserModel
     use SoftDeletingTrait;
 	protected $dates = ['deleted_at'];
 
+    public function company()
+    {
+        return $this->belongsTo('Company', 'company_id');
+    }
+
 
     /**
      * Returns the user full name, it simply concatenates