CommonClient: forget password when disconnecting (#3641)

* 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

* per request, adds handling on any disconnect to forget the saved password as to not leak it to other servers

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Co-authored-by: NewSoupVi <57900059+NewSoupVi@users.noreply.github.com>

kvui.py

@@ -596,6 +596,7 @@ class GameManager(App):
 
     def connect_button_action(self, button):
         self.ctx.username = None
+        self.ctx.password = None
         if self.ctx.server:
             async_start(self.ctx.disconnect())
         else: