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/oc.yml

@@ -353,7 +353,6 @@ oc:
     your_token: Vòstre geton d’accès
   auth:
     agreement_html: En vos marcar acceptatz <a href="%{rules_path}">las règlas de l’instància</a> e <a href="%{terms_path}">politica de confidencialitat</a>.
-    change_password: Seguretat
     delete_account: Suprimir lo compte
     delete_account_html: Se volètz suprimir vòstre compte, podètz <a href="%{path}">o far aquí</a>. Vos demandarem que confirmetz.
     didnt_get_confirmation: Avètz pas recebut las instruccions de confirmacion ?
@@ -369,6 +368,7 @@ oc:
     register: Se marcar
     resend_confirmation: Tornar mandar las instruccions de confirmacion
     reset_password: Reïnicializar lo senhal
+    security: Seguretat
     set_new_password: Picar un nòu senhal
   authorize_follow:
     error: O planhèm, i a agut una error al moment de cercar lo compte