Call update_device_chats automatically during configure (#1957)

update_device_chats() takes about 2 seconds on a modern device (Android) because the
welcome image file has to be written to the disk as a blob. The problem
was that this was done after the progress bar had vanished and before
anything else happened so that I thought that something had gone wrong
multiple times.

The UIs have to remove update_device_chats(), too..

python/src/deltachat/account.py

@@ -336,6 +336,9 @@ class Account(object):
     def get_deaddrop_chat(self):
         return Chat(self, const.DC_CHAT_ID_DEADDROP)
 
+    def get_device_chat(self):
+        return Contact(self, const.DC_CONTACT_ID_DEVICE).create_chat()
+
     def get_message_by_id(self, msg_id):
         """ return Message instance.
         :param msg_id: integer id of this message.