Enable Electron right click Inspect Element (#4003)

* upgrade to electron 2.0.18 and node 8.9.3

* enable Inspect Element in electron context menu

* use electron-context-menu 0.11.0 due to electron 4 requirement

packages/server/lib/gui/windows.coffee

@@ -209,9 +209,7 @@ module.exports = {
     if options.contextMenu
       ## adds context menu with copy, paste, inspect element, etc
       contextMenu({
-        ## don't show inspect element until this fix is released
-        ## and we upgrade electron: https://github.com/electron/electron/pull/8688
-        showInspectElement: false
+        showInspectElement: true
         window: win
       })