Skip mailer setup when Action Mailer is not loaded

When people are using 3rd-party services for delivering emails (e.g.
Mailgun), some choose not to load Action Mailer in their app, since it's
not used. Previously rodauth-rails would error when Action Mailer wasn't
loaded, so in this commit we make it work.

Closes #148

lib/generators/rodauth/install_generator.rb

@@ -63,6 +63,8 @@ def create_account_model
         end
 
         def create_mailer
+          return unless defined?(ActionMailer)
+
           template "app/mailers/rodauth_mailer.rb"
 
           MAILER_VIEWS.each do |view|

lib/generators/rodauth/templates/app/misc/rodauth_main.rb

@@ -56,6 +56,7 @@ class RodauthMain < Rodauth::Rails::Auth
     # Redirect to the app from login and registration pages if already logged in.
     # already_logged_in { redirect login_redirect }
 
+<% if defined?(ActionMailer) -%>
     # ==> Emails
     # Use a custom mailer for delivering authentication emails.
     create_reset_password_email do
@@ -81,6 +82,7 @@ class RodauthMain < Rodauth::Rails::Auth
       db.after_commit { email.deliver_later }
     end
 
+<% end -%>
     # ==> Flash
 <% unless json? || jwt? -%>
     # Match flash keys with ones already used in the Rails app.

lib/rodauth/rails/feature.rb

@@ -8,15 +8,15 @@ module Rodauth
     require "rodauth/rails/feature/callbacks"
     require "rodauth/rails/feature/csrf"
     require "rodauth/rails/feature/render"
-    require "rodauth/rails/feature/email"
+    require "rodauth/rails/feature/email" if defined?(ActionMailer)
     require "rodauth/rails/feature/instrumentation"
     require "rodauth/rails/feature/internal_request"
 
     include Rodauth::Rails::Feature::Base
     include Rodauth::Rails::Feature::Callbacks
     include Rodauth::Rails::Feature::Csrf
     include Rodauth::Rails::Feature::Render
-    include Rodauth::Rails::Feature::Email
+    include Rodauth::Rails::Feature::Email if defined?(ActionMailer)
     include Rodauth::Rails::Feature::Instrumentation
     include Rodauth::Rails::Feature::InternalRequest
   end

lib/rodauth/rails/feature/internal_request.rb

@@ -3,13 +3,13 @@ module Rails
     module Feature
       module InternalRequest
         def domain
-          return super unless missing_host?
+          return super unless missing_host? && rails_url_options
 
           rails_url_options.fetch(:host)
         end
 
         def base_url
-          return super unless missing_host? && domain
+          return super unless missing_host? && domain && rails_url_options
 
           scheme = rails_url_options[:protocol] || "http"
           port = rails_url_options[:port]
@@ -43,6 +43,8 @@ def missing_host?
         end
 
         def rails_url_options
+          return nil unless defined?(ActionMailer)
+
           ::Rails.application.config.action_mailer.default_url_options or
             fail Error, "There is no information to set the URL host from. Please set config.action_mailer.default_url_options in your Rails application, or configure #domain and #base_url in your Rodauth configuration."
         end