Add timestamp to SDBM RDF export

[demery]

Add a time stamp to the SDBM RDF export.

Change get_rdf.sh to add timetamp:

• get a list of files /usr/src/app/public/static/docs/output-*.ttl.gz => files_to_delete
• create new export named output-<TIMESTAMP>.ttl.gz
• export succeeds, delete all files in $files_to_delete

Change app/views/downloads/index.html

• list all RDF output files found in /usr/src/app/public/static/docs/