Persist resource across sessions

This was somehow lost once we implemented random resource strings
Although its good that initally a random resource is chosen, it
still should persist across sessions.

This helps to kill zombie client connections on the server because
most servers kill the old session once a new with the same resource
connects

Also dont notify the user about resource conflicts if no custom
resource is set. We should not expect users to know about resources
and what they are used for as long as possible.

gajim/common/connection.py

@@ -191,6 +191,7 @@ class CommonConnection:
                     'hostname': socket.gethostname(),
                     'rand': rand
             })
+            app.config.set_per('accounts', self.name, 'resource', resource)
         return resource
 
     def dispatch(self, event, data):

gajim/gui_interface.py

@@ -1162,10 +1162,18 @@ class Interface:
         account = obj.conn.name
         conn = obj.conn
         self.roster.send_status(account, 'offline', conn.status)
+
         def on_ok(new_resource):
             app.config.set_per('accounts', account, 'resource', new_resource)
             self.roster.send_status(account, conn.old_show, conn.status)
+
         proposed_resource = conn.server_resource
+        if proposed_resource.startswith('gajim.'):
+            # Dont notify the user about resource change if he didnt set
+            # a custom resource
+            on_ok('gajim.$rand')
+            return
+
         proposed_resource += app.config.get('gc_proposed_nick_char')
         dlg = dialogs.ResourceConflictDialog(_('Resource Conflict'),
             _('You are already connected to this account with the same '