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.yml

@@ -346,7 +346,6 @@ sr:
     your_token: Ваш приступни токен
   auth:
     agreement_html: Приступањем инстанци се слажете са <a href="%{rules_path}">правилима инстанце</a> и <a href="%{terms_path}">условима коришћења</a>.
-    change_password: Безбедност
     delete_account: Обриши налог
     delete_account_html: Ако желите да обришете Ваш налог, можете <a href="%{path}">наставити овде</a>. Бићете упитани да потврдите.
     didnt_get_confirmation: Нисте добили поруку са упутствима за потврду налога?
@@ -359,6 +358,7 @@ sr:
     register: Региструј се
     resend_confirmation: Пошаљи поруку са упутствима о потврди налога поново
     reset_password: Ресетуј лозинку
+    security: Безбедност
     set_new_password: Постави нову лозинку
   authorize_follow:
     error: Нажалост, десила се грешка при тражењу удаљеног налога