Description of problem:
There's not sanity checking, or defined rules for that matter, for what is acceptable in a username regarding underscores. A username of "______" is valid, but not desirable.
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. Go to zanata/account/register
2. Enter in a username of ______
3. tab to another field
No error shown
This case should not be acceptable.
The registration form can be completed, such that a new user with username "______" is created.
Verified at 1e5f1b1a87acca1b87a1f798a0c1e89d5c5263e3