* Add conditional to display "<username> is not registered" or Admin User profile in /admin/show.html.erb
-Displays a "<username> is not registered" message for unregistered users
-Displays all linked usernames and Admin User info for registered users
* Adds tests around unregistered users Admin User profiles
* Use predicate method in all conditionals in /admin/users/show.html.erb
* Adjusts message alerting admins that a user has not accepted the invite yet
- Makes the message displayed on the Admin User prof more explicit
- Makes GETS singular instead of plural in invitations_spec.rb
- Updates users_spec.rb to test against updated message