PLT-5458: If someone posts a channel link to channel_A that you don't belong to, it doesn't render properly (#7833)

* add channel link hints to post props

* optimization

* update regex, add unit test

* fix rebase issue

app/channel.go

@@ -757,6 +757,18 @@ func (a *App) GetChannelByName(channelName, teamId string) (*model.Channel, *mod
 	}
 }
 
+func (a *App) GetChannelsByNames(channelNames []string, teamId string) ([]*model.Channel, *model.AppError) {
+	if result := <-a.Srv.Store.Channel().GetByNames(teamId, channelNames, true); result.Err != nil && result.Err.Id == "store.sql_channel.get_by_name.missing.app_error" {
+		result.Err.StatusCode = http.StatusNotFound
+		return nil, result.Err
+	} else if result.Err != nil {
+		result.Err.StatusCode = http.StatusBadRequest
+		return nil, result.Err
+	} else {
+		return result.Data.([]*model.Channel), nil
+	}
+}
+
 func (a *App) GetChannelByNameForTeamName(channelName, teamName string) (*model.Channel, *model.AppError) {
 	var team *model.Team