specs: migrate from features to system specs

System specs have been available since Rails 5.1, and are better
integrated with the Rails framework.

- Rename `spec/features` to `spec/system`
- Rename `feature do` to `describe do`
- Configure Capybara for system specs

Steps mostly taken from https://medium.com/table-xi/a-quick-guide-to-rails-system-tests-in-rspec-b6e9e8a8b5f6

spec/system/users/dossier_shared_examples.rb

@@ -0,0 +1,41 @@
+RSpec.shared_examples 'the user can edit the submitted demande' do
+  scenario js: true do
+    visit dossier_path(dossier)
+
+    expect(page).to have_current_path(dossier_path(dossier))
+    click_on 'Demande'
+
+    expect(page).to have_current_path(demande_dossier_path(dossier))
+    click_on 'Modifier le dossier'
+
+    expect(page).to have_current_path(modifier_dossier_path(dossier))
+    fill_in('Texte obligatoire', with: 'Nouveau texte')
+    click_on 'Enregistrer les modifications du dossier'
+
+    expect(page).to have_current_path(demande_dossier_path(dossier))
+    expect(page).to have_content('Nouveau texte')
+  end
+end
+
+RSpec.shared_examples 'the user can send messages to the instructeur' do
+  let!(:commentaire) { create(:commentaire, dossier: dossier, email: 'instructeur@exemple.fr', body: 'Message envoyé à l’usager') }
+  let(:message_body) { 'Message envoyé à l’instructeur' }
+
+  scenario js: true do
+    visit dossier_path(dossier)
+
+    expect(page).to have_current_path(dossier_path(dossier))
+    click_on 'Messagerie'
+
+    expect(page).to have_current_path(messagerie_dossier_path(dossier))
+    expect(page).to have_content(commentaire.body)
+
+    fill_in 'commentaire_body', with: message_body
+    click_on 'Envoyer le message'
+
+    expect(page).to have_current_path(messagerie_dossier_path(dossier))
+    expect(page).to have_content('Message envoyé')
+    expect(page).to have_content(commentaire.body)
+    expect(page).to have_content(message_body)
+  end
+end