update and document GB postcode data

[mtmail]

Conversion script and documentation how the gb_postcode_data.sql file can be recreated. I used August data and you can download it from https://downloads.opencagedata.com/public/

The old file is a couple of year old. 92,639 postcodes are new. 83,512 no longer exist.

The documentation should be symlinked into /docs I'm waiting for #1195 to be merged first.

[lonvia]

Any chance we can do without the temporary table and do the projection in php instead? There must be a php binding for the proj library somewhere.

[mtmail]

I tested around and there's a PHP package allowing me to convert to WGS84 lat,lon. But the table schema currently has a geometry column and one can't directly INSERT FROM FILE into that. I'll try adding lat,lon columns and as last step have Postgresql fill the geometry column.

[mtmail]

Rebased. Projection is now done in PHP, thus no temporary table needed. Replaced geometry column with x and y columns (what we do for us-postcodes already). Added link to documentation in /docs sidebar.

[lonvia]

Superseded by #1252.