Fix #1326 Safari can't focus with draggable parent

Summary:
**Summary**

DraftEditors that lived inside draggable parents were useless in Safari. Now they work.
Solution found here: react-dnd/react-dnd#178 (comment)

**Test Plan**

Use the fiddle in #1326 with the updated version.
For a quick n dirty, just add this to the stylesheet:
`div[contenteditable="true"] { user-select: text;}`
Closes facebookarchive/draft-js#1356

Differential Revision: D5901625

fbshipit-source-id: 7d3797286e4040575b9b5503c776bff1e29b67a9

src/component/base/DraftEditor.react.js

@@ -225,6 +225,9 @@ class DraftEditor extends React.Component<DraftEditorProps, State> {
 
     const contentStyle = {
       outline: 'none',
+      // fix parent-draggable Safari bug. #1326
+      userSelect: 'text',
+      WebkitUserSelect: 'text',
       whiteSpace: 'pre-wrap',
       wordWrap: 'break-word',
     };