Refactor how filters are loaded from ENV

** Why are these changes being introduced:

The prior commit was getting flagged by CodeClimate for the complexity
of the extract_filters method.

** Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/gdt-128

** How does this address that need:

* This tries taking the processing of the ENV variable into its own
  method, to hopefully make extract_filters easier to read. The call to
  this processing is made on its own line, to only perform that task
  once (rather than doing it twice, on the following line of the method)

** Document any side effects to this change:

I may be overthinking this.

app/controllers/search_controller.rb

@@ -33,7 +33,7 @@ def extract_filters(response)
     aggs = response&.data&.search&.to_h&.dig('aggregations')
     return if aggs.blank?
 
-    active_filters = ENV.fetch('ACTIVE_FILTERS', '').split(',').map(&:strip)
+    active_filters = load_filters
 
     aggs = reorder_filters(aggs, active_filters) unless active_filters.blank?
 
@@ -50,10 +50,14 @@ def extract_results(response)
     response&.data&.search&.to_h&.dig('records')
   end
 
+  def load_filters
+    ENV.fetch('ACTIVE_FILTERS', '').split(',').map(&:strip)
+  end
+
   def reorder_filters(aggs, active_filters)
     aggs
-      .select { |key, value| active_filters.include?(key) }
-      .sort_by { |key, value| active_filters.index(key) }.to_h
+      .select { |key, _| active_filters.include?(key) }
+      .sort_by { |key, _| active_filters.index(key) }.to_h
   end
 
   def validate_q!