Update invitation process for our own User model

This was something that I forgot to do after substituting our own custom
User model in 95e0f72. The implementation of project member invitatioins
contained lots of references to UserProfile models and "old-style"
project membership attributes/methods. This updates the forms, views,
and template for project roster editing to reflect the new reality.

TODO still: make custom project roles with fine-grained permissions
(important)