Update `updated_at` on an issue when label or time spent was added

Summary

When labels on an issues changed or time spent was added it means the issue has changed somehow, but "updated_at" attribute on an issue is not updated.

Steps to reproduce

1. Create an issue
2. Check "updated_at" attribute via Rest Api (/api/v4/projects/:project_id/issues/:issue_iid)
3. Add spent time via comment /spend
4. Check "updated_at" attribute again - at this moment, you can see, that the "updated_at" was not changed

What is the expected correct behavior?

1. Create an issue
2. Check "updated_at" attribute via Rest Api (/api/v4/projects/:project_id/issues/:issue_iid)
3. Add spent time via comment /spend
4. Check "updated_at" attribute again - at this moment, we should see that "updated_at" has changed

Possible fixes

Update "updated_at" attribute when changing labels or time spent on an issue. Because this change means that the issue has changed.