Remove default instance of ClientWebView's PageState JS object

This default instance was causing double the number of preferred height
events, which was likely creating a race for ConversationWebView. This
might (should?) fix plain text email with quotes sometimes showing up
with a vastly incorrect height.

To ensure that a PageState is constructed properly, make ClientWebView
abstract and use replace the one direct use of the class in the accounts
editor for the signature with a new subclass.

See #283

ui/signature-web-view.js

@@ -0,0 +1,12 @@
+/*
+ * Copyright 2019 Michael Gratton <mike@vee.net>
+ *
+ * This software is licensed under the GNU Lesser General Public License
+ * (version 2.1 or later). See the COPYING file in this distribution.
+ */
+
+/**
+ * Application logic for SignatureWebView.
+ */
+
+var geary = new PageState();