Feature: Direct message from menu (#6956)

* Implement ability to send direct messages from the user menu

* Implement message warning users that direct messages are visible to all mentioned users

* Update locales

app/javascript/mastodon/locales/zh-CN.json

@@ -2,6 +2,7 @@
   "account.block": "屏蔽 @{name}",
   "account.block_domain": "隐藏来自 {domain} 的内容",
   "account.blocked": "Blocked",
+  "account.direct": "Direct Message @{name}",
   "account.disclaimer_full": "此处显示的信息可能不是全部内容。",
   "account.domain_blocked": "Domain hidden",
   "account.edit_profile": "修改个人资料",
@@ -56,6 +57,7 @@
   "column_header.unpin": "取消固定",
   "column_subheading.navigation": "导航",
   "column_subheading.settings": "设置",
+  "compose_form.direct_message_warning": "This toot will only be visible to all the mentioned users.",
   "compose_form.hashtag_warning": "这条嘟文被设置为“不公开”，因此它不会出现在任何话题标签的列表下。只有公开的嘟文才能通过话题标签进行搜索。",
   "compose_form.lock_disclaimer": "你的帐户没有{locked}。任何人都可以在关注你后立即查看仅关注者可见的嘟文。",
   "compose_form.lock_disclaimer.lock": "开启保护",