Custom broadcasts created directly via the API app won't have area
IDs since [1], where we started to distinguish between "names" (all
broadcasts have these) and IDs (for broadcasts created in this app).
We forgot to propagate the distinction into the code here.
This code fixes the bug for all broadcasts created after [1]. Any
custom broadcasts created before [1] will have their "ids" field set
instead of "names" so we won't show anything for them. This seems
reasonable as we don't support custom broadcasts yet.
[1]: 023a06d5fb
The aggregate names don't need to be sorted, but it helps to do
this for the tests, since the implementation of sets may not be
stable between machines and lead to sporadic test failures.
I did also toy with sorting granular area names so they have a
similar ordering, but this would take them out-of-order with IDs
and really the important thing is that the ordering is stable.
This applies some heuristics to try and keep the overall list of
areas short when many are selected in the same wider area.
Currently we only have relationship information between upper and
lower tier local authorities, so we can't / won't aggregate up to
Greater London (it's own special thing) or whole countries.
