Style "organisation select" as bootstrap 3 input

* Include formtastic-bootstrap and chosen CSS before maslow styles
(previously being included later which meant they needed higher
specificity)
* Override chosen styles to look like bootstrap input — box-shadow,
background, padding and border, using variables where possible
* Wrap in bootstrap grid to constrain width
* Tweak form-md-8 width to match a full-width input placed within a
col-md-8 grid