Insert markdown improvements, permissions flag

- Insert a markdown item before or after current focused item
- re-consolidate markdown on save
- if inserted item is escaped out of with no content, remove the item from the array as well as the DOM
- improvement for keyboard shortcut propagation
- add edit_permission boolean to ws_obj.state that controls insert, edit, and checkboxes (delete)
#628, #656
Remove leftover experiment
Adjust position of checkboxes, add "this worksheet is empty" message

#642
Use getState() when setting new state after saving an item

#628
Merge pull request #657 from codalab/feature/628-add-markdown-above-or-below

Feature/628 add markdown above or below