[MM-14719] convert users to bots through cmd (#10672)

* [MM-14719] convert users to bots through cmd

* address review comments, add / fix unit tests

* change command from "modify" to "convert"; review comments

* code review comments, clean up

app/bot.go

@@ -176,3 +176,12 @@ func (a *App) disableUserBots(userId string) *model.AppError {
 
 	return nil
 }
+
+// ConvertUserToBot converts a user to bot
+func (a *App) ConvertUserToBot(user *model.User) (*model.Bot, *model.AppError) {
+	result := <-a.Srv.Store.Bot().Save(model.BotFromUser(user))
+	if result.Err != nil {
+		return nil, result.Err
+	}
+	return result.Data.(*model.Bot), nil
+}