Simplify invite process.

Invite form now the same for both admin and non-admin.

Select the project, then click invite. This project cannot be changed in
the invite form.

Non-admin must invite to a project they lead.

Admin people can send an open invitation without specifying a project.
However nothing in Karaage points to this link at the present time.

Change-Id: I2d2d0773d197cb61d842215414887914431a0847