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/sr-Latn.yml

@@ -346,7 +346,6 @@ sr-Latn:
     your_token: Vaš pristupni token
   auth:
     agreement_html: Pristupanjem instanci se slažete sa <a href="%{rules_path}">pravilima instance</a> i <a href="%{terms_path}">uslovima korišćenja</a>.
-    change_password: Bezbednost
     delete_account: Obriši nalog
     delete_account_html: Ako želite da obrišete Vaš nalog, možete <a href="%{path}">nastaviti ovde</a>. Bićete upitani da potvrdite.
     didnt_get_confirmation: Niste dobili poruku sa uputstvima za potvrdu naloga?
@@ -359,6 +358,7 @@ sr-Latn:
     register: Registruj se
     resend_confirmation: Pošalji poruku sa uputstvima o potvrdi naloga ponovo
     reset_password: Resetuj lozinku
+    security: Bezbednost
     set_new_password: Postavi novu lozinku
   authorize_follow:
     error: Nažalost, desila se greška pri traženju udaljenog naloga