Enable export of BED/GFF files from text search

We need to provide a new way to download text search results in BED/GFF formats.

This functionality should be added to the export django app and be similar to how FASTA files are exported.

The BED and GFF data should be retrieved from the BED and GFF text files from the RNAcentral FTP archive. The code should:

[ ] check if the BED/FTP files are already saved locally, if not download them from FTP
[ ] figure out which file to look in using URS_taxids (or find a better strategy)
[ ] use grep or use a specialised tool to retrieve the corresponding entries
[ ] return the compressed file

The user interface should be the same as for the FASTA export. This functionality can also be reused for Galaxy integration.

RNAcentral / rnacentral-webcode