Extract out overrideable Catalog#display_home_content? method

It was previously hard-coded into catalog/index.html.erb that if you has_search_parameters?, you display search results, and otherwise you display "home" splash content.

But my app never wants to display home splash content. It wants to always display search results (with no search parameters, that would be: all results).

Other apps may want to have different criteria for when to display home splash screen results.

By making the logic rely on new extracted `display_home_content?` method, we provide an override hook point to easily customize this choice without having to override the template (with it's future compatibility and maintainance issues).

The default defintion of display_home_content? is `!has_search_parameters?` for complete backwards compatibility with previous logic.

app/controllers/concerns/blacklight/catalog.rb

@@ -16,6 +16,7 @@ module Blacklight::Catalog
     if respond_to? :helper_method
       helper_method :sms_mappings, :has_search_parameters?
       helper_method :search_facet_path
+      helper_method :display_home_content?
     end
 
     record_search_parameters

app/helpers/blacklight/catalog_helper_behavior.rb

@@ -206,6 +206,11 @@ def document_index_view_type query_params = params || {}
     end
   end
 
+  # Should we display special "home" splash screen content, instead of search results?
+  def display_splash_content?
+    !has_search_parameters?
+  end
+
   private
 
   # @param [String] format

app/views/catalog/index.html.erb

@@ -8,7 +8,7 @@
                                         view_config: conf) %>
 <% end %>
 
-<% unless has_search_parameters? %>
+<% if display_splash_content? %>
   <%# if there are no input/search related params, display the "home" partial -%>
   <%= render 'home' %>
   <%= render 'shared/sitelinks_search_box' %>

spec/views/catalog/index.html.erb_spec.rb

@@ -1,9 +1,9 @@
 # frozen_string_literal: true
 
 RSpec.describe "catalog/index.html.erb" do
-  describe "with no search parameters" do
+  describe "with splash content (no search parameters)" do
     before do
-      allow(view).to receive(:has_search_parameters?).and_return(false)
+      allow(view).to receive(:display_splash_content?).and_return(true)
       allow(view).to receive(:blacklight_config).and_return(CatalogController.blacklight_config)
       @response = instance_double(Blacklight::Solr::Response, empty?: true, total: 11, start: 1, limit_value: 10, aggregations: {})
     end
@@ -17,9 +17,9 @@
     end
   end
 
-  describe "with search parameters" do
+  describe "with results (search parameters)" do
     before do
-      allow(view).to receive(:has_search_parameters?).and_return(true)
+      allow(view).to receive(:display_splash_content?).and_return(false)
       stub_template "catalog/_results_pagination.html.erb" => ""
       stub_template "catalog/_search_header.html.erb" => "header_content"
       allow(view).to receive(:blacklight_config).and_return(Blacklight::Configuration.new)