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/fa.yml

@@ -344,7 +344,6 @@ fa:
     your_token: کد دسترسی شما
   auth:
     agreement_html: پیش از عضو شدن باید <a href="%{rules_path}">قوانین این سرور</a> و <a href="%{terms_path}">شرایط استفادهٔ</a> ما را بپذیرید.
-    change_password: امنیت
     delete_account: پاک‌کردن حساب
     delete_account_html: اگر می‌خواهید حساب خود را پاک کنید، از <a href="%{path}">این‌جا</a> پیش بروید. از شما درخواست تأیید خواهد شد.
     didnt_get_confirmation: راهنمایی برای تأیید را دریافت نکردید؟
@@ -357,6 +356,7 @@ fa:
     register: عضو شوید
     resend_confirmation: راهنمایی برای تأیید را دوباره بفرست
     reset_password: بازنشانی رمز
+    security: امنیت
     set_new_password: تعیین رمز تازه
   authorize_follow:
     error: متأسفانه حین یافتن آن حساب خطایی رخ داد