CommonClient: fix bug when using Connect button without a disconnect (#3609)

* makes the kivy connect button do the same username forgetting that /connect does to fix an issue where losing connection would make you unable to connect to a different server

* extract duplicate code

kvui.py

@@ -595,8 +595,8 @@ class GameManager(App):
                                              "!help for server commands.")
 
     def connect_button_action(self, button):
+        self.ctx.username = None
         if self.ctx.server:
-            self.ctx.username = None
             async_start(self.ctx.disconnect())
         else:
             async_start(self.ctx.connect(self.server_connect_bar.text.replace("/connect ", "")))