Introduce the external_files table

Introduce the external_files table

The external_files table is to be used by - for example -
security.d.o: we should export the files table to debian-security
under the disguised name of external_files.

We also introduce the use_extfiles setting, and a new check:
check.ExternalHashesCheck, which will compare the uploaded files
hashes to the external_files table, if that holds an entry for them.

It will only do that though, if the use_extfiles setting is on.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>