Provide sortable columns in private messages, similar to latest topics

[tobiaseigen]

Hey @eviltrout - thanks for adding column headers to messages. I like it!

I thought it was going to come with column sorting, like the typical latest view. I am just now trying to work through a huge backlog of messages as admin, and it would be very handy to be able to sort by which messages have had replies or been viewed.

I think this is mostly a feature for moderator or admin users who have a larger number of messages to manage, so I understand if it’s not a high priority. I can see via this handy data explorer query that it is only me as user 1 and discobot who have thousands of messages in our inbox, sent out automagically to new users. Ordinary civilians tend to have only a handful if any.

Filtering in this view would be helpful too at some stage, to allow us to bulk archive welcome messages that have no reply, for example.

[Hooksmith]

我们的论坛有一个用于处理私密社区问题的群组，通常不会一次性积累到 200 条消息。我们希望按顺序处理这些消息，但如果不保持标签页打开，就需要大量滚动（不过，即使保持标签页打开，收件箱也不会随着他人回复消息而实时更新，因此过一段时间后你仍然需要刷新以查看最新状态……）。

我知道 @sam 曾在此提到，一般建议是保持收件箱规模较小：

[Paid] Sortable group messages

FYI, our general use case is to be very strict about archiving. If you keep your team inbox reasonably clear there is very little need for these kind of features. Not strictly against column sorting in core, but its very rare to be needed. @carlokok do you tend to keep your “team inbox” reasonably clear as well?

我们完全赞同这一建议，但由于我们的用户活动量较大，且我们通常以批次方式处理这些问题，因此将收件箱规模保持较小对我们来说通常不可行（即 200 条消息的数量会在几天内迅速累积）。我们也不希望采用其他方式（例如电子邮件）来回答这些问题，因为我们希望让用户始终使用同一款产品，以简化操作并提升直观性。

如果私信中的列能像“最新”列表那样运作，那将非常棒：这样我们只需查看该群组，即可直接看到最旧的消息排在顶部，而无需先滚动到底部。使用公开分类并不符合我们的使用场景，因为我们需要确保消息内容仅在群组与用户之间保持私密。

实现这一功能需要多少工作量？是否欢迎提交相关 PR？