Tidy up over-large methods

Tidies up the `UnifiedSearchPresenter.facet_options` and
`FacetParameterParser.order` methods, as suggested by Dom Baggot in
review.  In his words:

    By breaking the steps down into pieces that do single things and
    avoiding mutation of shared variables it makes it easier to reason
    about each piece of behaviour, and makes it a lot more obvious where
    we could be extracting new methods to simplify further or allow
    reuse.