dshttpsettings: add field for cookies that should be kept

The ds proxy strips out cookies. Cookie names specified in
this field will not be stripped out. This is the frontend
change only.

public/app/features/plugins/partials/ds_http_settings.html

@@ -36,6 +36,18 @@
         </div>
       </div>
     </div>
+
+    <div class="gf-form-inline" ng-if="current.access=='proxy'">
+      <div class="gf-form max-width-30">
+        <span class="gf-form-label width-7">Cookies</span>
+        <bootstrap-tagsinput ng-model="current.jsonData.keepCookies" tagclass="label label-tag" placeholder="Add Name">
+        </bootstrap-tagsinput>
+        <info-popover mode="right-absolute">
+          Grafana Proxy deletes forwarded cookies by default. Specify cookies by name that should be forwarded to the data source.
+        </info-popover>
+      </div>
+    </div>
+
   </div>
 
   <h3 class="page-heading">HTTP Auth</h3>