PLT-3785 Re-enabled marking messages unread (#4179)

* Re-added ALT+Click to mark msgs unread, fixed mentions for DM

* Added string to /shortcuts

i18n/en.json

@@ -625,11 +625,11 @@
   },
   {
     "id": "api.command_shortcuts.msgs",
-    "translation": "#### Messages\n\nESC: Mark all messages in the current channel as read\nCTRL+UP (in empty input field): Reprint the previous message or slash command you entered\nCTRL+DOWN (in empty input field): Reprint the next message or slash command you entered\nUP (in empty input field): Edit your last message in the current channel\n@[character]+TAB: Autocomplete @username beginning with [character]\n:[character]+TAB: Autocomplete emoji beginning with [character]\n\n"
+    "translation": "#### Messages\n\n#### Messages\n\nALT+[click on a message]: Set a message as your oldest unread message in the current channel\nESC: Mark all messages in the current channel as read\nCTRL+UP (in empty input field): Reprint the previous message or slash command you entered\nCTRL+DOWN (in empty input field): Reprint the next message or slash command you entered\nUP (in empty input field): Edit your last message in the current channel\n@[character]+TAB: Autocomplete @username beginning with [character]\n:[character]+TAB: Autocomplete emoji beginning with [character]\n\n"
   },
   {
     "id": "api.command_shortcuts.msgs_mac",
-    "translation": "#### Messages\n\nESC: Mark all messages in the current channel as read\nCMD+UP (in empty input field): Reprint the previous message or slash command you entered\nCMD+DOWN (in empty input field): Reprint the next message or slash command you entered\nUP (in empty input field): Edit your last message in the current channel\n@[character]+TAB: Autocomplete @username beginning with [character]\n:[character]+TAB: Autocomplete emoji beginning with [character]\n\n"
+    "translation": "#### Messages\n\nALT+[click on a message]: Set a message as your oldest unread message in the current channel\nESC: Mark all messages in the current channel as read\nCMD+UP (in empty input field): Reprint the previous message or slash command you entered\nCMD+DOWN (in empty input field): Reprint the next message or slash command you entered\nUP (in empty input field): Edit your last message in the current channel\n@[character]+TAB: Autocomplete @username beginning with [character]\n:[character]+TAB: Autocomplete emoji beginning with [character]\n\n"
   },
   {
     "id": "api.command_shortcuts.name",