Add correct CSS for date picker

It was using a incomplete bootstrap-datepicker.css. I couldn't find a
better suited replacement, so just switched to an okay looking JQuery UI
theme.

Also switched to the minified version of JQuery UI.
Upgrade timepicker to current version

As part of reviewing datetime pickers, it seemed sensible to bump to the
current version.
Move timepicker JS load to dashboard's layout.html

@mbertheau correctly suggested as there's dashboard-global
initialisation going on in dashboard.js, loading the timepicker addon
should be global as well. That's more consistent with how we handle
other dashboard plugins as well.
Merge pull request #992 from tangentlabs/issue/992/datetimepicker

Fix datetime picker in dashboard