views: refactor the attachment/edit view

Make it more reusable, by:

- Renaming `attachment/update` to `attachment/edit`
- Refactoring the CSS styles into their own stylesheet
- Allow to specify the 'accept' option

spec/features/instructeurs/expert_spec.rb

@@ -108,7 +108,7 @@ feature 'Inviting an expert:' do
       expect(page).to have_text('Cet avis est confidentiel')
 
       fill_in 'avis_answer', with: 'Ma réponse d’expert : c’est un oui.'
-      find('.piece-justificative input[type=file]').attach_file(Rails.root + 'spec/fixtures/files/RIB.pdf')
+      find('.attachment input[type=file]').attach_file(Rails.root + 'spec/fixtures/files/RIB.pdf')
       click_on 'Envoyer votre avis'
 
       expect(page).to have_content('Votre réponse est enregistrée')