deform2: sequence_item.pt fixups

Here are a couple fix-ups for sequence_item.pt.

The sequence layout still needs further love.  I'm not particularly fond of the 'panel' layout (in particular I find
the panel-head and panel-footer to be ugly) but right now I haven't really got a better suggestion.

Anyhow, this is a start.  This pull addresses:
- 'row' (not 'container') is the class for the column container in bs3
- add 'form-group' class to provide vertical margin between sequence items
- 'col-xs-*' instead of 'col-md-*' so that close button and drag handle always get their own column