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/pt.yml

@@ -353,7 +353,6 @@ pt:
     your_token: O teu token de acesso
   auth:
     agreement_html: Registando-te concordas em seguir <a href="%{rules_path}">as regras da instância</a> e <a href="%{terms_path}">os nossos termos de serviço</a>.
-    change_password: Alterar palavra-passe
     confirm_email: Confirmar e-mail
     delete_account: Eliminar conta
     delete_account_html: Se desejas eliminar a conta, podes <a href="%{path}">continua aqui</a>. Uma confirmação será pedida.
@@ -367,6 +366,7 @@ pt:
     register: Registar
     resend_confirmation: Reenviar instruções de confirmação
     reset_password: Criar nova palavra-passe
+    security: Alterar palavra-passe
     set_new_password: Editar palavra-passe
   authorize_follow:
     error: Infelizmente, ocorreu um erro ao buscar a conta remota