FilterView should allow filtering on rating

[fire-eggs]

Can't filter on rating (safe, questionable, etc) using filterView.

Rating doesn't apply to the tags, but can be applied to the images. [The count of images for the tag wouldn't necessarily match the count of images shown.]

[fire-eggs]

I forgot [because it is completely un-intuitive!]: the rating control in postsFilterView will affect which images are shown!

1. Need to add rating controls
2. Need to pass rating to callback in addition to tag
3. In tagview.py, filterCall / producer extended to use rating from filterView instead of postsFilterView