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-CN.yml

@@ -351,7 +351,6 @@ zh-CN:
     your_token: 你的访问令牌
   auth:
     agreement_html: 注册即表示你同意遵守<a href="%{rules_path}">本实例的相关规定</a>和<a href="%{terms_path}">我们的使用条款</a>。
-    change_password: 帐户安全
     delete_account: 删除帐户
     delete_account_html: 如果你想删除你的帐户，请<a href="%{path}">点击这里继续</a>。你需要确认你的操作。
     didnt_get_confirmation: 没有收到确认邮件？
@@ -364,6 +363,7 @@ zh-CN:
     register: 注册
     resend_confirmation: 重新发送确认邮件
     reset_password: 重置密码
+    security: 帐户安全
     set_new_password: 设置新密码
   authorize_follow:
     error: 对不起，寻找这个跨站用户时出错