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-TW.json

@@ -2,6 +2,7 @@
   "account.block": "封鎖 @{name}",
   "account.block_domain": "隱藏來自 {domain} 的一切貼文",
   "account.blocked": "Blocked",
+  "account.direct": "Direct Message @{name}",
   "account.disclaimer_full": "下列資料不一定完整。",
   "account.domain_blocked": "Domain hidden",
   "account.edit_profile": "編輯用者資訊",
@@ -56,6 +57,7 @@
   "column_header.unpin": "取下",
   "column_subheading.navigation": "瀏覽",
   "column_subheading.settings": "設定",
+  "compose_form.direct_message_warning": "This toot will only be visible to all the mentioned users.",
   "compose_form.hashtag_warning": "This toot won't be listed under any hashtag as it is unlisted. Only public toots can be searched by hashtag.",
   "compose_form.lock_disclaimer": "你的帳號沒有{locked}。任何人都可以關注你，看到發給關注者的貼文。",
   "compose_form.lock_disclaimer.lock": "上鎖",