Content Views Rework: filter updates for api consistency and to support CLI

This commit has a few minor updates to ensure api consistency, support
CLI and improve behavior such as:
- routes: use content view resource on filter index/create
- routes: do not use content view resource on filter show/update/delete
- only update filter parameter created_at if the filter have changed
- updates to docs to  support apipie bindings for hammer-cli-katello support
- ...