Add a multi-select checkbox widget
Add a multi-select checkbox widget

This widget renders a SelectMultiField (also a QuerySelectMultipleField)
as a list of checkboxes with bootstrap makeup.

fixes #250

Signed-off-by: Jan Losinski <577c4104c61edf9f052c616c0c23e67bef4a9955@wh2.tu-dresden.de>

lukasjuhrich/pycroft,lukasjuhrich/pycroft,agdsn/pycroft,agdsn/pycroft,agdsn/pycroft,agdsn/pycroft,lukasjuhrich/pycroft,agdsn/pycroft,lukasjuhrich/pycroft