Stable URLs should be used for BIA access

[zephyris]

Previously, we used:
https://ftp.ebi.ac.uk/pub/databases/biostudies/S-BIAD1/

This got changed to:
https://ftp.ebi.ac.uk/biostudies/fire/S-BIAD/

We need to make sure that https://www.ebi.ac.uk/biostudies/files// is the correct basis for everything. Looks like they might be treating some files differently and then the way we handle the root URL is broken if we give it the canonical https://www.ebi.ac.uk/biostudies/files/ as root.