refs TryGhost/Team#1071
Specific tier visibility for a post was previously stored in `visibility` column directly, which had a limitation of 50 charts. Going forward, for the specific tiers visibility of post/page, we use `tiers` array in API that contains list of tiers with access for post. This change -
- replaces `filter` type to `tiers` for visibility
- adds new visibility filter segment component in post settings menu which stores array of tiers instead of nql string for visibility
- updates serializer and model for post/page
- updates tests