display timestamp of osm world data

[hixi]

to see when the last update was made inside the world database.

[das-g]

• relevant PBF format details incl. What are the replication fields for?

We can probably extract the timestamp with osmupdate planet.pbf --out-timestamp

[das-g]

I think this should be made visible both

• somewhere on the homepage (maybe on the export forms for existing and new excerpts)
• somewhere in the downloaded package or its name (or both)
  • currently, we use the export time for naming the ZIP archive and some of its contents. Maybe we should use the PBF timestamp for that, instead?