User and Group names should be unique

If a user and group exist with the same name, you may encounter some bad side effects. To reproduce:

• Create a user, e.g. "reviewers"
• Create a group, e.g. "reviewers"
• Add "reviewers" (the user) as a member of "reviewers" (the group).

Problems that are easy to reproduce:

• You can no longer view the User properties from the Admin Center. Clicking the link from the User list just displays a spinner.
• If you view the Group, the Members list will contain the Group itself instead of the User. Clicking the member link takes you to the Group.

Problems that are not easy to reproduce:

• On a POC server we had a problem where a user logged in during such a situation was unable to access the "Dashboard" tab under "Home". This caused an error with a stack trace (I did not save the stack trace unfortunately!). The error was about permissions. We suspect the problem was caused by the Tasks widget because we also had the Tasks widget displayed in a document tab and the user was unable to access this tab as well (same stack trace). The user did have a task pending. I tried to reproduce this locally but was not successful.

After deleting the offending groups on the POC server, the problem was resolved.

It may be worth mentioning that the POC server was using nuxeo-multi-tenant as well.