Koenig - Standardise on <em> but allow <i> to be toggled off via toolbar

refs TryGhost/Ghost#9505
- switch toolbar `I` button to toggle `<em>` instead of `<i>`, this standardises it with the <kbd>Meta+I</kbd> shortcut
- if the active markup is `<i>` instead of `<em>` then highlight the `I` button and allow it to be toggled off to prevent confusion

lib/koenig-editor/addon/components/koenig-toolbar.js

@@ -98,6 +98,10 @@ export default Component.extend({
 
     actions: {
         toggleMarkup(markupName) {
+            if (markupName === 'em' && this.get('activeMarkupTagNames.isI')) {
+                markupName = 'i';
+            }
+
             this.toggleMarkup(markupName);
         },
 

lib/koenig-editor/addon/templates/components/koenig-toolbar.hbs

@@ -10,8 +10,8 @@
 <button
     type="button"
     title="Italic"
-    class="koenig-toolbar-btn koenig-toolbar-btn-italic {{if activeMarkupTagNames.isI "selected"}}"
-    {{action "toggleMarkup" "i"}}
+    class="koenig-toolbar-btn koenig-toolbar-btn-italic {{if (or activeMarkupTagNames.isEm activeMarkupTagNames.isI) "selected"}}"
+    {{action "toggleMarkup" "em"}}
 >
     I
 </button>