Fix issue description syntax highlighting and math rendering

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/33877

Regressed in
https://gitlab.com/gitlab-org/gitlab-ce/commit/5a95d6f8dae00b31b694759c6ddbf6d83b1a7890#3a15290e7696397415523e0b664aceb3dd4010ae_0_46
where the ref no longer matches what we call `renderGFM` on.

app/assets/javascripts/issue_show/components/description.vue

@@ -53,7 +53,7 @@
     },
     methods: {
       renderGFM() {
-        $(this.$refs['gfm-entry-content']).renderGFM();
+        $(this.$refs['gfm-content']).renderGFM();
 
         if (this.canUpdate) {
           // eslint-disable-next-line no-new

spec/javascripts/issue_show/components/description_spec.js

@@ -96,4 +96,20 @@ describe('Description component', () => {
       });
     });
   });
+
+  it('applies syntax highlighting and math when description changed', (done) => {
+    spyOn(vm, 'renderGFM').and.callThrough();
+    spyOn($.prototype, 'renderGFM').and.callThrough();
+    vm.descriptionHtml = 'changed';
+
+    Vue.nextTick(() => {
+      setTimeout(() => {
+        expect(vm.$refs['gfm-content']).toBeDefined();
+        expect(vm.renderGFM).toHaveBeenCalled();
+        expect($.prototype.renderGFM).toHaveBeenCalled();
+
+        done();
+      });
+    });
+  });
 });