Add headings to the security settings page (#6661)

* Changes the headings' rank of the security settings section

This commit changes the existing headings' rank of the security settings section from level 6 to level 4.

* Renames the auth.change_password string into auth.security

The "Security" preferences' section used to be called "Change password". When it was renamed, the string name wasn't changed.
This commits changes auth.change_password to auth.security.

* Adds a heading to the password change form

There was previously no heading for the part of the "Security" page that contain the password change form.
This commit adds a rank 4 heading to this section and reintroduces an "auth.change_password" string to be used inside it.

* Removes useless HR elements

The various sections of the "Security" settings page were previously separated by HR elements.
Now that there is proper headings, they're not required anymore.

* Updates CSS

This commit updates CSS in such a way that the same style is applied to all the H4 elements of the settings.

* Correct a mistake

A character went missing on one of the previous commits, broking the CSS.
This new commit fixes it.

config/locales/fr.yml

@@ -359,7 +359,6 @@ fr:
     your_token: Votre jeton d’accès
   auth:
     agreement_html: En vous inscrivant, vous souscrivez <a href="%{rules_path}">aux règles de l’instance</a> et à <a href="%{terms_path}">nos conditions d’utilisation</a>.
-    change_password: Sécurité
     confirm_email: Confirmer mon adresse mail
     delete_account: Supprimer le compte
     delete_account_html: Si vous désirez supprimer votre compte, vous pouvez <a href="%{path}">cliquer ici</a>. Il vous sera demandé de confirmer cette action.
@@ -377,6 +376,7 @@ fr:
     register: S’inscrire
     resend_confirmation: Envoyer à nouveau les consignes de confirmation
     reset_password: Réinitialiser le mot de passe
+    security: Sécurité
     set_new_password: Définir le nouveau mot de passe
   authorize_follow:
     error: Malheureusement, il y a eu une erreur en cherchant les détails du compte distant