fixed erroneous warning about wrong nights on hotel assignments page

People kept seeing the warning message, "This attendee has requested setup/teardown and you are assigning them to a regular room, wnich will automatically decline their request to help with setup/teardown."

Which is fine except that they'd see it even when it didn't apply!  The backend did the correct thing, so if you ignored the warning it wouldn't mess anything up, which is good.  This pull request makes the warning only happen when it should.
This is the kind of function which would be really easy to unit test, which would have caught this bug.  One of these days I'll write a bunch of unit tests for these Angular apps.  One of these days...