Ticket/253

Resolves #253, I think?

I added form-level errors to the top of housing, event, and person form templates. Those are the ones that I think are sufficiently long to merit them and they all also redirect back to the same form on either successful or unsuccessful submission.

Notably, though, this pull req does not: add error messages to other forms or add success messages to any forms (though the person form view adds one for those forms through the messages framework), but I kinda think we're fine without.