style: signin button styled accordingly to UX

app/views/layouts/_header.haml

@@ -38,7 +38,7 @@
                 - if request.path == new_user_registration_path
                   %li
                     .fr-hidden-sm.fr-unhidden-lg.fr-link--sm= t('views.shared.account.already_user_question')
-                %li= link_to t('views.shared.account.signin'), new_user_session_path, class: "fr-btn"
+                %li= link_to t('views.shared.account.signin'), new_user_session_path, class: "fr-btn fr-btn--tertiary fr-icon-account-circle-fill fr-btn--icon-left"
 
               %li
                 - if dossier.present? && nav_bar_profile == :user

config/locales/views/shared/fr.yml

@@ -19,4 +19,4 @@ fr:
         already_user_question: 'Vous avez déjà un compte ?'
         already_user: 'J’ai déjà un compte'
         create: 'Créer un compte'
-        signin: 'Connexion'
+        signin: 'Se connecter'

spec/system/sessions/sign_in_spec.rb

@@ -4,7 +4,7 @@ describe 'Signin in:' do
 
   scenario 'an existing user can sign-in' do
     visit root_path
-    click_on 'Connexion'
+    click_on 'Se connecter', match: :first
 
     sign_in_with user.email, 'invalid-password'
     expect(page).to have_content 'Courriel ou mot de passe incorrect.'
@@ -16,7 +16,7 @@ describe 'Signin in:' do
 
   scenario 'an existing user can lock its account' do
     visit root_path
-    click_on 'Connexion'
+    click_on 'Se connecter', match: :first
 
     5.times { sign_in_with user.email, 'bad password' }
     expect(user.reload.access_locked?).to be false
@@ -60,7 +60,7 @@ describe 'Signin in:' do
     # For now only test the default behavior (an error message is displayed).
     scenario 'they get an error message' do
       visit root_path
-      click_on 'Connexion'
+      click_on 'Se connecter', match: :first
 
       sign_in_with user.email, password
       expect(page).to have_content('Vous devez confirmer votre compte par courriel.')

spec/system/users/managing_password_spec.rb

@@ -6,7 +6,7 @@ describe 'Managing password:', js: true do
     scenario 'a simple user can reset their password' do
       visit root_path
       within('.fr-header .fr-container .fr-header__tools .fr-btns-group') do
-        click_on 'Connexion'
+        click_on 'Se connecter'
       end
       click_on 'Mot de passe oublié ?'
       expect(page).to have_current_path(new_user_password_path)
@@ -37,7 +37,7 @@ describe 'Managing password:', js: true do
     scenario 'an admin can reset their password', js: true do
       visit root_path
       within('.fr-header .fr-container .fr-header__tools .fr-btns-group') do
-        click_on 'Connexion'
+        click_on 'Se connecter'
       end
       click_on 'Mot de passe oublié ?'
       expect(page).to have_current_path(new_user_password_path)