Forcing a unique login per user row, making sure code can handle it

[pkamps]

Background
The DB table ezuser allows non-unique values in the login column. That obviously does not make any sense. This pull request is changing the DB schema forcing unique values in the login column. It also adjusts the kernel code which is required to avoid fatal errors creating users in the admin backend.

Testing Instructions

1. Checkout branch
2. Update the DB schema: php update/run.php -u ezpub -p ezpub -d ezpub
3. Login to the admin UI
4. Create a new user account and send for publishing
5. Create another new user account - make sure to pick exactly the same value for the 'username' field.
6. Confirm that you don't get a 500 fatal error. You should see an input validation error message, saying that the username already exists.