[BUGFIX] Improve student validation for late days allowed

[weastel]

Please check if the PR fulfills these requirements:

• The PR title and message follows our guidelines

What is the current behavior?

Fixes #3449

What is the new behavior?

Invalid Student ID Error is shown when student selected is not from the course.

Other information?

1. Go to Late Days Allowed in Development
2. Try to insert late days for ta2(ta2 is not in Development but in Sample)
3. Invalid Student ID error should be shown

[bmcutler]

This boolean name seems inverse to me.
IsUserInCourse should be true if the query is non-empty.