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/ca.yml

@@ -359,7 +359,6 @@ ca:
     your_token: El teu identificador d'accés
   auth:
     agreement_html: En inscriure't, acceptes seguir <a href="%{rules_path}">els nostres termes del servei</a> i <a href="%{terms_path}">la nostra política de privadesa</a>.
-    change_password: Seguretat
     confirm_email: Confirmar correu electrònic
     delete_account: Suprimeix el compte
     delete_account_html: Si vols suprimir el compte pots <a href="%{path}">fer-ho aquí</a>. Se't demanarà confirmació.
@@ -378,6 +377,7 @@ ca:
     register: Registre
     resend_confirmation: Torna a enviar el correu de confirmació
     reset_password: Restableix la contrasenya
+    security: Seguretat
     set_new_password: Estableix una contrasenya nova
   authorize_follow:
     error: Malauradament, ha ocorregut un error cercant el compte remot