Lab validation back-end fix

This PR turned out longer and less focused than I intended so I don't expect everything to be accepted.

What does this mr do?

Makes sure exam labs are properly validated. Closes: #345 (closed)

Actions taken to fix bug

I've added 2 ways to verify a lab.

1. Through LabValidator. This makes sure that a bad @Validated Lab form get's stopped at the web layer during the BindingResult validation.
2. Through a custom ConstraintValidator.

...
@ValidExamLab
public class Lab implements Serializable {
...

This makes sure that upon any Lab entity's lifecyle events it gets validated and exam labs don't have null percentage, otherwise throws a ConstraintValidationException. This should make sure no bad labs get persisted while keeping the @Nullable annotation.

I noticed that Lab has a custom getter that returns 0 if the examLabPercentage is null. I've removed this since it breaks validation. Do you maybe need a custom serializer for this?

If you like this validation pattern I can expand it to other classes who may have similar issues.

Does this MR meet the acceptance criteria?

• I have added a changelog entry to reflect the significant changes I made and the bug I fixed.
• A test was created to test the bug.
• I have updated the documentation accordingly.
• I adhere to the style guide.