Because we need to pass a Ruby hash to Rabl instead of a JSON string,
get rid of our serialiser and instead turn it into a function that just
converts the distance & time units.

Fix a bug in the test that had the `'1-2'` key as a symbol instead of a
string which was caused by a copy-paste from JSON and not being thorough
enough in search-and-replace.

Refs #6203

This serialiser will take `Route#costs` and convert the distance and
time fields from meters to kilometres and seconds to minutes
respectively. We need this because the frontend uses kilometre and
minute units while the TomTom API gives us the others (and we store the
data we get from TomTom without treatment).

Unfortunately, due to the way that Rabl works, this doesn't quite work
just yet. The serializer returns a string, and Rabl just puts this
string into the JSON output instead of a real JSON hash. Looks like I'm
going to have to convert my serializer into a generic converter.

Refs #6203

Render the `costs` field of a `Chouette::Route` as JSON. This lives at:
http://stif-boiv.dev:3000/referentials/4/lines/1857/routes/7/costs.json

I want this in order to get the distance & time costs of a route so that
I can merge them with `JourneyPattern#costs` on the frontend on
`JourneyPatternsCollection#show`.

Refs #6203

Subtleties were that the links need to reference `StopArea`s. In
particular, the `edit` link goes to the `StopAreaReferential` path
instead of the referential-StopArea path.

Refs #5586