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/zh-HK.yml

@@ -156,7 +156,6 @@ zh-HK:
   applications:
     invalid_url: 所提供的網址不正確
   auth:
-    change_password: 登入資訊
     didnt_get_confirmation: 沒有收到確認指示電郵？
     forgot_password: 忘記了密碼？
     login: 登入
@@ -164,6 +163,7 @@ zh-HK:
     register: 登記
     resend_confirmation: 重發確認指示電郵
     reset_password: 重設密碼
+    security: 登入資訊
     set_new_password: 設定新密碼
   authorize_follow:
     error: 對不起，尋找這個跨站用戶的過程發生錯誤