Post-process usernames from LDAP/AD before getting/creating User objects.
LDAP and AD usernames have basically no rules, while django usernames have lots

of rules. This change makes it so people log in using the username that they

normally have, but the username stored in Review Board will have all the junk

stripped out.
This fixes numerous issues that people have encountered, such as:

- Case insensitivity would cause multiple users to be created in Review Board

- Usernames with spaces or special characters would cause URL resolution and

  form validation failures, since the db backend didn't enforce the rules but

  our URL regexes and form fields did.

Ran unit tests.