service to return longitude/latitude against Jersey postcodes

[eatyourpeas]

Similar to #1106

E12 currently looks up postcodes against an API which returns longitude and latitude, and these are stored in the model (using SRID 27700) and this is used to plot them on the scatter plots. There is no authoritative open source lookup of Jersey postcodes matched with geocodes. The Jersey government have 'private partners' who sell this data at £85 a year as a csv download is the cheapest I can find. Or there is Google maps API or a string of other providers such as OpenCage, but these are similarly paid for services. There is not much about this when I search, but generally people in Jersey seem to say that accuracy and coverage is not that great. For the moment therefore a hacky work around is not to save geocode data on save and skip the lookups if the postcode is a Jersey one.

To address this issue we will need either a datasource of postcodes that map to geocodes or pay for a service that does this.