models: render attestations in a simpler way

The older method of instanciating an entire new rendering stack can be
made simpler using Rails >= 5.0 methods.

See https://api.rubyonrails.org/classes/ActionController/Renderer.html#method-i-render

app/models/attestation_template.rb

@@ -109,8 +109,11 @@ class AttestationTemplate < ApplicationRecord
 
   def build_pdf(dossier)
     attestation = render_attributes_for(dossier: dossier)
-    action_view = ActionView::Base.new(ActionController::Base.view_paths, attestation: attestation)
-    attestation_view = action_view.render(file: 'new_administrateur/attestation_templates/show', formats: [:pdf])
+    attestation_view = ApplicationController.render(
+      file: 'new_administrateur/attestation_templates/show',
+      formats: :pdf,
+      assigns: { attestation: attestation }
+    )
 
     StringIO.new(attestation_view)
   end