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/gl.yml

@@ -359,7 +359,6 @@ gl:
     your_token: O seu testemuño de acceso
   auth:
     agreement_html: Rexistrándose acorda seguir <a href="%{rules_path}">as normas da instancia</a> e <a href="%{terms_path}">os termos do servizo</a>.
-    change_password: Seguridade
     confirm_email: Confirmar correo-e
     delete_account: Eliminar conta
     delete_account_html: Se desexa eliminar a súa conta, pode <a href="%{path}">facelo aquí</a>. Pediráselle confirmación.
@@ -378,6 +377,7 @@ gl:
     register: Rexistro
     resend_confirmation: Voltar a enviar intruccións de confirmación
     reset_password: Restablecer contrasinal
+    security: Seguridade
     set_new_password: Establecer novo contrasinal
   authorize_follow:
     error: Desgraciadamente, algo fallou ao buscar a conta remota