Fixes some drag-n-drop problems (facebookarchive#1599)

Summary:
**Summary**
Fixes issue facebookarchive#1383. Fixes problem which is: onDrop breaks onSelect/onChange events.
Fixes issue facebookarchive#1454 (partial). Fixes problem which is: after first internal drop dragged text will not be moved to target position, but copied.
The solution is to set `editor._internalDrag` to false not during onDragOver events, but after end of drag-n-drop.

**Test Plan**
Attached reproducable videos (on Chrome for Linux)
Issue facebookarchive#1383 before and after:
![1383-before](https://user-images.githubusercontent.com/3238637/46585340-68ec4100-ca78-11e8-9c2f-c98e7cb381ff.gif)
![1383-after](https://user-images.githubusercontent.com/3238637/46585344-6c7fc800-ca78-11e8-9c2e-fdac5594d1d0.gif)
Issue facebookarchive#1454 before and after:
![1454-before](https://user-images.githubusercontent.com/3238637/46585364-9e912a00-ca78-11e8-9c17-5dfcec5a2e16.gif)
![1454-after](https://user-images.githubusercontent.com/3238637/46585361-9a650c80-ca78-11e8-8b7f-dac6c6af4b23.gif)
Pull Request resolved: facebookarchive#1599

Differential Revision: D10236073

fbshipit-source-id: 3b7b816630f3b5b15931cb53782a1d5b9e9d5121

src/component/handlers/drag/DraftEditorDragHandler.js

@@ -121,11 +121,7 @@ const DraftEditorDragHandler = {
       editor.update(moveText(editorState, dropSelection));
     } else {
       editor.update(
-        insertTextAtSelection(
-          editorState,
-          dropSelection,
-          (data.getText(): any),
-        ),
+        insertTextAtSelection(editorState, dropSelection, (data.getText(): any)),
       );
     }
     endDrag(editor);
@@ -138,14 +134,6 @@ const DraftEditorDragHandler = {
   onDragOver: function onDragOver(editor: DraftEditor, e: Object): void {
     e.preventDefault();
   },
-
-  /**
-   * Edge disallows drag event by default (A No Symbol (⃠) will be shown on the dragged image by default).
-   * Without preventing the default dragOver handler a drop event can not be triggered.
-   */
-  onDragOver: function onDragOver(editor: DraftEditor, e: Object): void {
-    e.preventDefault();
-  },
 };
 
 function endDrag(editor) {