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 tag filter_helpers to View::Mappings::Filter
Inline Filter#by_tag? and #by_tags?

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

* Didn't need a field for mappings
* consistency with local variable names in tag add/remove
Better comment for View::Mappings::Filter
Separate QS hash manipulation into own class
filter_by_type_path -> Filter::Query#by_type
remove_filter_by_type_path -> Filter::Query#remove_by_type
Move sort methods from helper

* To Filter::Query
Reorder View::Mappings::Filter fields

* Make it a little easier to see what's where at a glance
Filter knows about fields it needs to pass through

* Should enable last part of https://www.pivotaltracker.com/story/show/70586196, i.e
  "The non-JS version doesn't keep your parameters when you go back to the filter form"