Optimise packs display

#462 
Add num_sounds and last_updated to the packs table so that we don't have to compute aggregate queries each time we show packs.

Update the values initially like this (this will regenerate all license files):

```
from django.db import transaction
ps = Pack.objects.all()
l = ps.count()
with transaction.commit_on_success():
    for i, p in enumerate(ps, 1):
        print "%s/%s" % (i, l)
        p.process()
```