Rough edges around very large histories

This concerns the SARS-CoV-2 histories, for instance https://usegalaxy.org/u/sars-cov2-bot/h/2020-05-02-update-with-snpeff-fix
- [ ] import of shared history times out with a 504:
- and no indication that the request likely succeeded anyway
- [ ] export to file of history also times out before revealing download link

Start by reproducing the import and export flows with the linked SARS-CoV-2 history and observe the requests that end in a 504. Trace the history import and export entry points, then verify that completed imports communicate success and exports reveal a download link instead of timing out.

Written by the indexing model from the issue text.