[Bug 1050565] AoA twitter feed filter

Allows you to add users to ignore via the AoA tool on the moderate page. This replaces CC_IGNORE_LIST in settings.py.
@rehandalal Could I get a second opinion on how this is being done? Previously I had ban and ignore separated out, so this way is much more generic than before. That being said, it is also more ugly and possibly less maintainable than before. Maybe I'm thinking too far into it though, thoughts?