searchable: refactor search_attributes helper methods into class.

Move the `search_attributes` helper methods inside the Searchable
concern into a SearchContext helper class. This is so we don't have to
pass `params` and `current_user` down through a bunch of different
helper methods, and so that these private helper methods don't pollute
the object's namespace.

test/unit/concerns/searchable.rb

@@ -19,7 +19,7 @@ class SearchableTest < ActiveSupport::TestCase
     context "for a nonexistent attribute" do
       should "raise an error" do
         assert_raises(ArgumentError) do
-          Post.search_attribute(:answer, 42, User.anonymous)
+          Post.search_attributes({ answer: 42 }, :answer)
         end
       end
     end