Build export download links from locale keys (#7271)

Build export download links from locale keys (#7271)

The four bulk download links on the export page had their URLs
hardcoded in the view, with the Overpass one spelt out a second
time in export.js so the bbox could be appended on each map move.

Follow the pattern already used by the help page and drive the list
from locale keys, adding a URL key alongside the existing title and
description for each source. This removes the duplication between
the view and the JS, and allows the URLs to be localised or pointed
elsewhere by deployments that are not openstreetmap.org.

export.js no longer needs to know the Overpass URL: it sets the
search component of the link's existing href rather than rebuilding
the whole thing.

The wiki link now points to Downloading_data, the target of the
redirect that the previous /wiki/Download URL followed.

settings.overpass_url is deliberately untouched: it is the query
server's interpreter endpoint, a separate concern from the export
download link.