gui: remember the ambient temperature in the user config

Persists whenever set via the entry, and re-applies automatically on
connect - so it survives a server restart (which would otherwise fall
back to config.json's ambient_temperature) without retyping it.

client/brewpi_gui.py

@@ -382,6 +382,11 @@ class Window(QtWidgets.QMainWindow, Ui_MainWindow):
 		# on_sud_changed() routes the reply to update_sud_forecast() instead
 		# of a save dialog.
 		self.msg_sud.send({'Save': True})
+		# Re-apply the last ambient temperature we set, in case the server
+		# restarted since (and so fell back to its own config.json default).
+		remembered_ambient = self.user_config.get('ambient_temperature')
+		if remembered_ambient is not None:
+			self.msg_system.send({'AmbientTemp': remembered_ambient})
 
 	def on_plot_timer(self):
 		self.plot.sample(
@@ -517,6 +522,7 @@ class Window(QtWidgets.QMainWindow, Ui_MainWindow):
 			value = float(self.lineEdit_ambient.text())
 		except ValueError:
 			return
+		self.user_config.set('ambient_temperature', value)
 		self.msg_system.send({'AmbientTemp': value})
 
 	def update_status_env_label(self):