Add the SSID count to the devices table

[boveus]

I tried to do an SSID count column, which was an N+1 query and crashed my dev VM. We should probably try to store this value in the DB and have it calculated as we ingest the CSVs.

Once this is done, we can add an SSID count to the device index view. I think that would be a good piece of info to have on there.