Wire Sud upload/download into the brewpi_gui client

Adds File menu actions to save the running Sud's schedule to a local
sud.json and to replace it from one, via the Sud websocket channel's
new Download/Upload messages. File dialogs are opened up front on the
GUI thread; the actual read/write happens off the websocket recv
callback, which runs on a worker thread and must not touch Qt itself.

main_window.py is regenerated from brewpi.ui via pyuic5 5.15.11, which
also picks up an unrelated pre-existing drift (the default URI had
fallen out of sync with the .ui source).

client/brewpi.ui

@@ -3675,6 +3675,8 @@
     <property name="title">
      <string>&amp;File</string>
     </property>
+    <addaction name="actionSudDownload"/>
+    <addaction name="actionSudUpload"/>
    </widget>
    <widget class="QMenu" name="menu_Help">
     <property name="title">
@@ -3726,6 +3728,22 @@
     <string>Stop</string>
    </property>
   </action>
+  <action name="actionSudDownload">
+   <property name="text">
+    <string>&amp;Download Sud...</string>
+   </property>
+   <property name="toolTip">
+    <string>Save the running Sud's schedule to a local sud.json</string>
+   </property>
+  </action>
+  <action name="actionSudUpload">
+   <property name="text">
+    <string>&amp;Upload Sud...</string>
+   </property>
+   <property name="toolTip">
+    <string>Replace the running Sud's schedule with a local sud.json</string>
+   </property>
+  </action>
  </widget>
  <resources/>
  <connections/>