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/ko.yml

@@ -361,7 +361,6 @@ ko:
     your_token: 액세스 토큰
   auth:
     agreement_html: 이 등록으로 <a href="%{rules_path}">이용규약</a> 과 <a href="%{terms_path}">약관</a>에 동의하는 것으로 간주됩니다.
-    change_password: 보안
     confirm_email: 확인 메일 승인
     delete_account: 계정 삭제
     delete_account_html: 계정을 삭제하고 싶은 경우, <a href="%{path}">여기서</a> 삭제할 수 있습니다. 삭제 전 확인 화면이 표시됩니다.
@@ -377,6 +376,7 @@ ko:
     register: 등록하기
     resend_confirmation: 확인 메일을 다시 보내기
     reset_password: 비밀번호 재설정
+    security: 보안
     set_new_password: 새 비밀번호
   authorize_follow:
     error: 리모트 계정을 확인하는 도중 오류가 발생했습니다