Allow self-activation after signing required user agreements.
Do not self-activate, or show user agreements to, an uninvited user:
i.e., one who is neither invited by default according to site policy,
nor a member of the "All users" group.

refs #1706
Store correct collection uuid in signature/click link.

closes #1706
Support user agreement viewing and signing in Workbench.

refs #1699
Divert processing when user agreements are needed, instead of just
omitting page content from layout.

closes #1699
Merge branch '1694-user-agreement'