allow issue date adjustments from gantt chart

Allows dragging issues horizontally in the gantt chart and adds buttons
to change start or due date.

Note: Only works properly after reverting ff9471679199ff173169dee265a4d29b06aa2a10
"Replace the JS calendar with a jQueryUI version." so far.

This is a port of the original redmine patch: http://www.redmine.org/issues/2024