lnurl: make requests async, don't block Qt GUI, rm LUD-16 support

- in lnurl.py, make request methods async
- in Qt GUI, lnurl network requests no longer block the GUI thread
  - but they still do in the kivy GUI
- "lightning address" (LUD-16) support is removed for now as the
  email addresses are indistinguishable from openalias email addresses
  (both protocols should have added and enforced a prefix, or similar,
   to remove this kind of ambiguity -- now we would need to make a
   network request just to identify what kind of ID we were given)

electrum/gui/qt/completion_text_edit.py

@@ -81,6 +81,8 @@ class CompletionTextEdit(ButtonsTextEdit):
             return
 
         QPlainTextEdit.keyPressEvent(self, e)
+        if self.isReadOnly():  # if field became read-only *after* keyPress, exit now
+            return
 
         ctrlOrShift = bool(e.modifiers() & (Qt.ControlModifier | Qt.ShiftModifier))
         if self.completer is None or (ctrlOrShift and not e.text()):