Refactor filter and sort

* Move lots of fields out of `MappingsController`
* Move lots of methods out of `FilterHelper`
* Use a single `View::Mappings::Filter.new(@site, params)` object instead to generate the `#mappings` scope
* `Filter` has `QueryParams` that can be used to modify a single parameter of the current filter & sort operation in the view like `@filter.query.with_tag('2i')` or `@filter.sort_by_path`