Better logic on content filtering in cogs using `feeds_core`

[geirawsm]

Filter logic should be:

1. If only allow filter, deny all other items
2. If only deny filter, only deny items with these filters
3. If both filters are activated, allow first, then deny. becomes the intersection of a Venn-diagram

Documentation should reflect this to avoid misunderstanding

[geirawsm]

737b771