Insert block menu (#297)

* Insert block behavior

* Functional with horizontal rule

* Add list and image upload working

* Cleanup typing

* Closest to correct behavior so far

* Improve block insert on list

* Hide (+) after clicking menu item

* Bad merge

frontend/components/Editor/plugins/EditList.js

@@ -0,0 +1,7 @@
+// @flow
+import EditList from 'slate-edit-list';
+
+export default EditList({
+  types: ['ordered-list', 'bulleted-list', 'todo-list'],
+  typeItem: 'list-item',
+});