Explicitly select which adbusters to support

[mjangda]

Sites should have to select which adbusters they want served and any others should return a 403 error. No point in serving adbusters that a VIP doesn't want/need.

[natebot]

This PR allows selection via a filter but it doesn't return a 403. I opted to just return out of the function like it does elsewhere, which results in a 404.

Were you thinking that a control panel could be provided for managing the adbusters? If so, is there a plugin that you'd suggest I use as an example?