make paste focuses the pasted cell

This is to make paste behave in the same manner that insert cell above
and below currently work (inserting a new item causes that item to
receive focus).

This solves a problem where, if a user pastes below a cell whose bottom
edge is below the current viewing area, they have no indication that any
action has occurred. The same applies for paste above a cell whose top
edge is occluded.

IPython/html/static/notebook/js/notebook.js

@@ -1040,6 +1040,7 @@ var IPython = (function (IPython) {
             var cell_data = this.clipboard;
             var new_cell = this.insert_cell_above(cell_data.cell_type);
             new_cell.fromJSON(cell_data);
+            new_cell.focus_cell();
         };
     };
 
@@ -1053,6 +1054,7 @@ var IPython = (function (IPython) {
             var cell_data = this.clipboard;
             var new_cell = this.insert_cell_below(cell_data.cell_type);
             new_cell.fromJSON(cell_data);
+            new_cell.focus_cell();
         };
     };