Save the latest callout card emoji

refs https://github.com/TryGhost/Team/issues/1206

- Re-use the saved emoji as the default for future callout cards
- Introduced a localstorage util that ignores errors. Ignoring errors avoids issues with browsers that don't support localstorage

ghost/admin/app/utils/localstorage.js

@@ -0,0 +1,27 @@
+/**
+ * Returns the value for a localstorage key.
+ * Silently ignores errors.
+ * @param {string} key
+ * @returns {string?} The value
+ */
+export function get(key) {
+    try {
+        return localStorage.getItem(key);
+    } catch (e) {
+        // do nothing here
+    }
+}
+
+/**
+ * Sets a localstorage value at the key `key`.
+ * Silently ignores errors.
+ * @param {string} key The key where to store the value
+ * @param {string} value The value to store
+ */
+export function set(key, value) {
+    try {
+        localStorage.setItem(key, value);
+    } catch (e) {
+        // do nothing here
+    }
+}

ghost/admin/lib/koenig-editor/addon/components/koenig-card-callout.js

@@ -1,3 +1,4 @@
+import * as storage from '../../../../app/utils/localstorage';
 import Browser from 'mobiledoc-kit/utils/browser';
 import Component from '@glimmer/component';
 import {EmojiButton} from '@joeattardi/emoji-button';
@@ -7,6 +8,8 @@ import {run} from '@ember/runloop';
 import {inject as service} from '@ember/service';
 import {set} from '@ember/object';
 
+const storageKey = 'gh-kg-callout-emoji';
+
 export default class KoenigCardCalloutComponent extends Component {
     @service config;
     @service feature;
@@ -56,7 +59,7 @@ export default class KoenigCardCalloutComponent extends Component {
         this.args.registerComponent(this);
 
         const payloadDefaults = {
-            calloutEmoji: '💡',
+            calloutEmoji: storage.get(storageKey) || '💡',
             calloutText: '',
             backgroundColor: 'grey'
         };
@@ -94,6 +97,7 @@ export default class KoenigCardCalloutComponent extends Component {
     @action
     setCalloutEmoji(emoji) {
         this._updatePayloadAttr('calloutEmoji', emoji);
+        storage.set(storageKey, emoji);
     }
 
     @action