Continued implementation of attachment drag+drop

Cannot get working in chrome reliably due to conflicting handling of
events and drag+drop API. Getting attachment drop working breaks other
parts of TinyMCE.
Implementing current work as should still work for MD editor and within
FireFox.

Related to #1460

resources/js/components/markdown-editor.js

@@ -256,7 +256,7 @@ class MarkdownEditor {
             }
 
             const clipboard = new Clipboard(event.dataTransfer);
-            if (clipboard.hasItems()) {
+            if (clipboard.hasItems() && clipboard.getImages().length > 0) {
                 const cursorPos = cm.coordsChar({left: event.pageX, top: event.pageY});
                 cm.setCursor(cursorPos);
                 event.stopPropagation();

resources/js/components/sortable-list.js

@@ -21,11 +21,9 @@ class SortableList {
                 this.$emit('sort', {ids: sortable.toArray()});
             },
             setData(dataTransferItem, dragEl) {
-                console.log('cat');
                 const jsonContent = dragEl.getAttribute('data-drag-content');
                 if (jsonContent) {
                     const contentByType = JSON.parse(jsonContent);
-                    dataTransferItem.setData('bookstack/json', jsonContent);
                     for (const [type, content] of Object.entries(contentByType)) {
                         dataTransferItem.setData(type, content);
                     }

resources/js/components/wysiwyg-editor.js

@@ -638,8 +638,8 @@ class WysiwygEditor {
 
                 });
 
+                // Custom drop event handling
                 editor.on('drop', function (event) {
-                    console.log('drop')
                     let dom = editor.dom,
                         rng = tinymce.dom.RangeUtils.getCaretRangeFromPoint(event.clientX, event.clientY, editor.getDoc());