Infrastructure and changes to the Query Tool for realtime preference handling. Refs #3294

Highlights of this patch include:
- Changes will affect SQL Editors in Create dialog boxes, SQL tab of the main screen, Query tool, History entries in the query tool, Query tool opened in New Tab/Window
- All the components of SQL editor will refer to single source of preferences which is cached in the Browser object. All other redundant ajax get preference calls are removed.
- SQL editor will not refer template JS variables anymore, once all the references are removed the template variables will also be removed.
- Code refactoring wherever possible.
- Covered JS test cases wherever possible.

web/pgadmin/browser/static/css/browser.css

@@ -1,3 +1,6 @@
+:root {
+  --codemirror-font-size : 1em;
+}
 /* Styles for the main browser */
 .browser-pane-container {
     position: absolute;
@@ -66,3 +69,7 @@ samp,
 .pg-login-icon {
   font-size: 16px;
 }
+
+.CodeMirror {
+  font-size: var(--codemirror-font-size, '1em');
+}