Various fixes

Fix OAuth and Realtime issues introduced in 9a515b9234

[DATABASE] Fix an empty default value mistake introduced in
fde929b151

[DATABASE][PostgreSQL] Avoid use of pg_constraint.consrc, which was removed in
PostgreSQL 12.

[DATABASE][MariaDB] Fix a typo introduced in aed2344bd4

[DAEMON] Wrap an assignment inside "switch":
a follow-up to adc689cb15

plugins/Poll/classes/User_poll_prefs.php

@@ -41,7 +41,7 @@ class User_poll_prefs extends Managed_DataObject
     public $user_id;          // int id
     public $hide_responses;   // bool
     public $created;          // datetime
-    public $modified;         // datetime
+    public $modified;         // timestamp
 
     /**
      * The One True Thingy that must be defined and declared.