Search for a postcode should redirect to /postcode/

[dracos]

Currently it does only an MP search (which works fine) and then a normal search (which I assume always will return no results).
If we recognise the search term is a postcode it should redirect to postcode which will either redirect to MP page or show MSPs etc.
This would fix #951 and #1184 (though not #1562).

[dracos]

Fixed in aea839b