Allow indentation beyond one level deeper than block above.

Summary:
**Summary**

Solves facebookarchive/draft-js#270. Removes restriction on indenting beyond one level deeper than the block above. Also allows indenting the very first item in a list. And all I had to do was delete code!

![image](https://cloud.githubusercontent.com/assets/253298/19991316/8dfb376e-a1f1-11e6-8e93-b8b49c30010b.png)

**Test Plan**

1. Try indenting the first item in a list.
2. Try indenting a list item more than one level deeper than the item above it.
Closes facebookarchive/draft-js#762

Differential Revision: D7137964

fbshipit-source-id: 597732b363537e34a0f466c3188f334b3fa30a9e

Author: aforismesen

src/model/modifier/RichTextEditorUtil.js

@@ -210,28 +210,11 @@ const RichTextEditorUtil = {
 
     event.preventDefault();
 
-    // Only allow indenting one level beyond the block above, and only if
-    // the block above is a list item as well.
-    var blockAbove = content.getBlockBefore(key);
-    if (!blockAbove) {
-      return editorState;
-    }
-
-    var typeAbove = blockAbove.getType();
-    if (
-      typeAbove !== 'unordered-list-item' &&
-      typeAbove !== 'ordered-list-item'
-    ) {
-      return editorState;
-    }
-
     var depth = block.getDepth();
     if (!event.shiftKey && depth === maxDepth) {
       return editorState;
     }
 
-    maxDepth = Math.min(blockAbove.getDepth() + 1, maxDepth);
-
     var withAdjustment = adjustBlockDepthForContentState(
       content,
       selection,