Add a basic feature to sort mappings by priority

- made-up numbers in the table, all the same
- not actually doing any sorting
- sort option only shows up on the non-js filter page
Remove extraneous space
Add a 'scopes' block in mapping_spec

* Move the only other scope tests to it
Add a with_traffic_summary scope to Mapping

* has counts (no percentages)
Sort by hit_count when sorting by traffic

- displayed hit counts and percentages are still fake
- hit column is still always there (not hidden when sorting by path)
- still no javascript sorting option
Only show hits column when sorting by traffic
Show real hit counts for mappings

(percentages are still fake)
Add a have_hit_counts matcher
Show real percentages
Display hit counts with commas
Use reduce to sum an array

This is clearer than inject(0).
Add rudimentary sort-by with js
Make js dropdown work properly for sorting mappings
Fix failing features

We now have two 'Path' links within .filters, so we need to disambiguate
them in the step definition.
Show the link to remove all filters when sorting by traffic
Add failing test for mapping scope interactions
Update acts-as-taggable-on from 3.0.1 to 3.1.1

* This fixes the issue where the Mapping.with_traffic_summary scope
  would not chain with Mapping.tagged_with, generating invalid SQL when
  a `count` was required (falling over on `any?` and the like)
* While we initially patched AATO ourselves, it turns out a fix was
  already merged for AATO 3.1.0
  (https://github.com/mbleigh/acts-as-taggable-on/pull/417)
* 3.1.1 also had a tag counting performance improvement, which in
  truth doesn't do much for us but requires a migration
Don't lose sorting when filtering by path or new URL
Rename 'traffic' to 'hits' in the context of sorting

We already refer to hits and analytics, and introducing a third name
for the same thing is likely to be more confusing.
Add helpers to display hit counts and percentages