Webpack all the things! Fixes #2135

This significantly speeds up loading of the application; in an average of 3 tests, v1.6
loaded in 11.5s in the runtime on a Mac, whilst the webpacked version of the code
loaded in 5.53s.

web/pgadmin/static/css/codemirror.overrides.css

@@ -68,4 +68,8 @@
   width: 100%;
   padding-left: 5px;
   position: absolute;
+}
+
+.CodeMirror-gutters {
+  z-index: 2;
 }