Only use helper_method when the view layer is present

In API mode, the view layer is not available and these trigger
method_missing errors. This fix is similar to how devise solved this
problem: heartcombo/devise#3732

.rubocop_todo.yml

@@ -108,7 +108,7 @@ Metrics/MethodLength:
 # Offense count: 9
 # Configuration parameters: CountComments.
 Metrics/ModuleLength:
-  Max: 208
+  Max: 211
   Exclude:
     - 'lib/blacklight/solr/search_builder_behavior.rb'
 

app/controllers/concerns/blacklight/catalog.rb

@@ -9,7 +9,9 @@ module Blacklight::Catalog
   # The following code is executed when someone includes blacklight::catalog in their
   # own controller.
   included do
-    helper_method :sms_mappings, :has_search_parameters?
+    if respond_to? :helper_method
+      helper_method :sms_mappings, :has_search_parameters?
+    end
 
     helper Blacklight::Facet
 

app/controllers/concerns/blacklight/controller.rb

@@ -14,17 +14,20 @@ module Blacklight::Controller
 
     # now in application.rb file under config.filter_parameters
     # filter_parameter_logging :password, :password_confirmation
-    helper_method :current_user_session, :current_user, :current_or_guest_user
     after_action :discard_flash_if_xhr
 
     # handle basic authorization exception with #access_denied
     rescue_from Blacklight::Exceptions::AccessDenied, :with => :access_denied
 
-    # extra head content
-    helper_method :has_user_authentication_provider?
-    helper_method :blacklight_config, :blacklight_configuration_context
-    helper_method :search_action_url, :search_action_path, :search_facet_url, :search_facet_path
-    helper_method :search_state
+    if respond_to? :helper_method
+      helper_method :current_user_session, :current_user, :current_or_guest_user
+
+      # extra head content
+      helper_method :has_user_authentication_provider?
+      helper_method :blacklight_config, :blacklight_configuration_context
+      helper_method :search_action_url, :search_action_path, :search_facet_path
+      helper_method :search_state
+    end
 
     # Specify which class to use for the search state. You can subclass SearchState if you
     # want to override any of the methods (e.g. SearchState#url_for_document)

app/controllers/concerns/blacklight/search_context.rb

@@ -2,10 +2,12 @@
 module Blacklight::SearchContext
   extend ActiveSupport::Concern
 
-  # The following code is executed when someone includes blacklight::catalog::search_session in their
+  # The following code is executed when someone includes Blacklight::Catalog::SearchSession in their
   # own controller.
-  included do  
-    helper_method :current_search_session, :search_session
+  included do
+    if respond_to? :helper_method
+      helper_method :current_search_session, :search_session
+    end
   end
 
   module ClassMethods

app/controllers/concerns/blacklight/token_based_user.rb

@@ -3,7 +3,9 @@ module Blacklight::TokenBasedUser
   extend ActiveSupport::Concern
 
   included do
-    helper_method :encrypt_user_id
+    if respond_to? :helper_method
+      helper_method :encrypt_user_id
+    end
 
     rescue_from Blacklight::Exceptions::ExpiredSessionToken do
       head :unauthorized