Add a type filter

* When filter selected its title updates, nothing is shown in the
panel-body
* Options are all, redirect and archive
* There’s a known problem when searching by New URL and type - New URL
requires mappings to be redirects - selecting archive in this instance
is ignored
Move tag filters to filter helper

* Keep filter logic in one place
Show how mappings are sorted

* Later this can become an option, eg sort by date edited or hits
Move filter menus into partials

* Rename existing path and new url partials to indicate they are form
fields
Link to type filtered mappings from dashboard