luci-base: fix click and drag handling

luci-base: fix click and drag handling

Previously when a table has the sortable property true, the
whole row was draggable but without any useful effect on desktop
or mobile. Only commencing the drag from the drag button worked
as intended. This interfered with text selections or other actions
in the table row.

Now the drag and touch events are bound to the drag button only.
The result is the same but the row contents are now selectable.

This change works on both desktop and touch only devices like
mobile.

Signed-off-by: Paul Donald <redacted>