importer: Create an extra table for feeds

importer: Create an extra table for feeds

We are currently using the overrides table to import any third-party
feeds. That creates the problem that those tables can only hold one row
for each AS which we cannot easily merge without losing the source of
the information.

Therefore this patch creates a new table which allow us to store this
information per feed and we will use that information if there is no
overrides information.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>