Suggest a user ID when an admin is creating a new user

The same way it's done when a user is registering.

refs #1361
Fix redirects after changing registrations path to /users/registrations

Routes on /users/registrations were not being ignored on redirects.
Also changed the solution to #888: now there's a "blacklist" of routes on
SessionLocalesController for URLs we should not GET.

refs #1361
refs #888
Remove unnecessary rescue_from from UsersController

refs #1361