Add Vue app structure for integration overrides

Only init the app for now

app/assets/javascripts/integrations/overrides/components/integration_overrides.vue

+<script>
+export default {
+  name: 'IntegrationOverrides',
+  props: {
+    overridesPath: {
+      type: String,
+      required: true,
+    },
+  },
+};
+</script>
+
+<template>
+  <div></div>
+</template>

app/assets/javascripts/integrations/overrides/index.js

+import Vue from 'vue';
+import IntegrationOverrides from './components/integration_overrides.vue';
+
+export default () => {
+  const el = document.querySelector('.js-vue-integration-overrides');
+
+  if (!el) {
+    return null;
+  }
+
+  const { overridesPath } = el.dataset;
+
+  return new Vue({
+    el,
+    render(createElement) {
+      return createElement(IntegrationOverrides, {
+        props: {
+          overridesPath,
+        },
+      });
+    },
+  });
+};

app/assets/javascripts/pages/admin/integrations/overrides/index.js

+import initIntegrationOverrides from '~/integrations/overrides';
+
+initIntegrationOverrides();