#676 checkbox for toggling edit mode on a worksheet

- Only enabled if user has edit permissions. Otherwise checkbox is disabled and displays "editing not allowed"
- If editing is on, checkboxes for items are displayed, and the insert, edit, move, and delete commands are enabled
#676 only allow reordering items if editing mode is on
Merge pull request #677 from codalab/feature/676-toggle-editing

Feature/676 toggle editing