EventType::getIdentityTypeEntityFormMode() should return a form mode string

[MegaChriz]

On the event type form, when you select an alternative form mode for a person, it looks like the change is not saved, because when the form is reloaded the default form mode seems to be selected.

I have found that the bug is in EventType::getIdentityTypeEntityFormMode(). This returns a boolean, while it should return a string representing the selected form mode, according to EntityTypeInterface::getIdentityTypeEntityFormMode().

[MegaChriz]

I have added an automated test covering EventType::getIdentityTypeEntityFormMode().

[freelock]

I've merged this over on freelock/rng.