Koenig - Fixed backspace not deleting empty para at beginning of doc

refs https://github.com/TryGhost/Ghost/issues/9311
- something that became apparent after adding the "<kbd>Enter</kbd> in post title adds blank paragraph" feature was that using that feature left you in a position where Backspace doesn't do what you expect - it does nothing rather than deleting the blank paragraph
- added logic to the `handleBackspaceKey` handler in `{{koenig-editor}}` to detect when backspace is pressed on a blank paragraph at the start of the doc so that we remove it then focus the title

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

@@ -528,6 +528,20 @@ export default Component.extend({
             return;
         }
 
+        // if the caret is at the beginning of the doc, on a blank para, and
+        // there are more sections then delete the para and trigger the
+        // `cursorDidExitAtTop` closure action
+        let isFirstSection = section === section.parent.sections.head;
+        if (isFirstSection && isCollapsed && offset === 0 && (section.isBlank || section.text === '') && section.next) {
+            this.editor.run((postEditor) => {
+                postEditor.removeSection(section);
+            });
+
+            // allow default behaviour which will trigger `cursorDidChange` and
+            // fire our `cursorDidExitAtTop` action
+            return;
+        }
+
         // if the section about to be deleted by a backspace is a card then
         // actually delete the card rather than selecting it.
         // However, if the current paragraph is blank then delete the paragraph