Reduce intermittent spec failures by making VueJS resource interceptor

decrement outstanding resource counts when HTTP response is received

Before the count would be reduced 500 ms after a DOM update tick, which could
cause race conditions since the `DatabaseCleaner` could run in the middle of
a Rails controller handling the response.

Partial fix to #21197 and other intermittent spec failures.

app/assets/javascripts/boards/vue_resource_interceptor.js.es6

-Vue.http.interceptors.push((request, next)  => {
+Vue.http.interceptors.push((request, next) => {
   Vue.activeResources = Vue.activeResources ? Vue.activeResources + 1 : 1;
-  Vue.nextTick(() => {
-    setTimeout(() => {
-      Vue.activeResources--;
-    }, 500);
+  next(function (response) {
+    Vue.activeResources--;
   });
-  next();
 });