Enable browser caching of static resources

This sends the Cache-Control header to allow caching of static resources,
and checks the If-Modified-Since header to allow browser to use the
cached resource on refresh. Also fixes some paths that caused redirects
(core//foo -> core/foo)

gui/syncthing/core/upgradingDialogDirective.js

@@ -2,6 +2,6 @@ angular.module('syncthing.core')
     .directive('upgradingDialog', function () {
         return {
             restrict: 'A',
-            templateUrl: 'syncthing/core//upgradingDialogView.html'
+            templateUrl: 'syncthing/core/upgradingDialogView.html'
         };
 });