Fix the tests

* Where @filter could be nil, use #try
* Unrecognised filter-by-types shouldn't count to Filter#active?
* Sorting by hits should count to Filter#active?
Move trailing slash to canonical_filter

* This was in the controller
* It didn't need to be, this is a job for the filter
Move much of filter_helper to View::Mappings::Filter
Minor cleanups

* Didn't need a field for mappings
* consistency with local variable names in tag add/remove
Inline Filter#by_tag? and #by_tags?

* Use the collection methods #any? and #include? instead