活动摘要邮件选项，可包含所有活动

[Falco]

该插件提供以下功能：

Restore Mailing List Mode Daily Summary

For a variety of reasons the option for mailing list mode users to receive a daily summary email was removed. It turns out there are a handful of communities that relied on this feature and as such it provoked a good discussion about the restoration of the feature. It was mentioned in that topic that I was working on this. Thanks to @lkramer and team for sponsoring this work! https://github.com/joebuhlig/discourse-mlm-daily-summary This plugin adds an option to the email preferences for…