[PLT-6697] Added CTRL/CMD+SHIFT+K shortcut to open 'more direct messages' dialog (#6534)

* add keypress shortcut to open 'more direct messages' dialog

* removed duplicate e.preventDefault()

* add text description to /shortcuts output

app/command_shortcuts.go

@@ -47,6 +47,7 @@ func (me *ShortcutsProvider) DoCommand(args *model.CommandArgs, message string)
 		"api.command_shortcuts.nav.unread_next",
 		"api.command_shortcuts.nav.switcher",
 		"api.command_shortcuts.nav.switcher_team",
+		"api.command_shortcuts.nav.direct_messages_menu",
 		"api.command_shortcuts.nav.settings",
 		"api.command_shortcuts.nav.recent_mentions",
 		// Files shortcuts