Focus add issuable form input when expanded

See https://gitlab.com/gitlab-org/gitlab-ce/issues/34336

app/assets/javascripts/issuable/related_issues/components/add_issuable_form.vue

@@ -84,12 +84,15 @@ export default {
 
   mounted() {
     const $input = $(this.$refs.input);
+
     new GfmAutoComplete(this.autoCompleteSources).setup($input, {
       issues: true,
     });
     $input.on('shown-issues.atwho', this.onAutoCompleteToggled.bind(this, true));
     $input.on('hidden-issues.atwho', this.onAutoCompleteToggled.bind(this, false));
     $input.on('inserted-issues.atwho', this.onInput);
+
+    this.$refs.input.focus();
   },
 
   beforeDestroy() {