Fix for Issue #1343

Fixed a bug where moving between "pages" on a spreadsheet would leave the user unable to enter marks. This was because an action was bound to each grade box, and this action would send the entered values to the db. Moving between pages would generate new grade boxes with actions not bound to them- the fix is to watch the table holding the grade boxes for changes and bind actions to newly created boxes. 

There is a second bug included in this issue where moving between grade boxes after entering a grade would make the client raise an alert. This was because grades_table.rjs was attempting to update a cell that did not exist. The solution to this is to have a JavaScript function check for the existence of the cell before attempting to update it.