Hide generation date unless export is downloadable
Hide generation date unless export is downloadable

When the export is being processed, but not yet online,
it would show the generation date along with
"Export in progress", which is not logical.

That’s because the generation date is set at the starting
time of the export.

Refs #1609.

Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2