Program user unique indices

[swalker2m]

Per ShortCut 3151, changes the uniqueness constraints on program user / user links such that:

• A user cannot occupy the same role twice in the same program.
• A user cannot be more than one of PI, CoI, CoI-RO in the same program.
• A user can be one of Pi, CoI, CoI-RO and at the same time a support user.
• A user can be both the primary and secondary support for a program.

[swalker2m]

Note, this is an addendum to #1522.

[swalker2m]

This will simply allow multiple ProgramUser entries to be associated with the same User. Obviously that's annoying because all the attributes (names, email) are duplicated. Another approach would allow one ProgramUser to have multiple roles. That's more complex, but I could switch to that if necessary.