Filter local authorities data to active English units

[matt-dray]

#52 highlights that some codes in the data can't be matched to a name becuase the codes are inactive. The data also has non-English codes in it, while the currently-used lookup is England only.

@DCEW to confirm: it's only English LAs in scope, right?

[matt-dray]

Actually, seems okay for now: only the named LAs are appearing in the dropdown of the deployed app. Also, some of this issue may be better dealt with in the LA pipeline. Labelling as 'won't' and moving to backlog for now.