The JavaScript validation doesn't pass if we have more than two decimal
places (I think because it uses a number field step value of 0.01).
Round the values to allow them to pass frontend validation.

Refs #6203

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

Calculate distance and time cost between Route stops. Refs #6095 

We say `TomTom` is disabled when no API key is present. If this is the
case, the `after_save` callback that uses it shouldn't be executed.

I had to change my `API_KEY` constant to an instance variable to be able
to change it for testing.

Refs #6095