Invitation support

[evert]

This might be a sub/superset of #113 , but we have a more specific requirement for this now.

The flow we'd like to use to onboard a new user is as follows:

1. "create a new user"
2. Make sure an email address is entered.
3. Checkbox for 'send invitation email' on this page.
4. Url field (that can accept any url) that the user will go to after they've completed the invitation process
5. (nice to have, but also complicated): list of permissions the user will initially have.

If 3 is checked, we automatically send a user an invitation email. This email will contain a link back to a12nserver where they will need to enter a new password. After they've done so, they will be automatically redirected to the uri of step 4, OR if this was not entered, to the a12n-server standard login screen.

This interface does not need to be pretty yet.