Implement independent username for SSO

[Stuart Metcalfe]

Currently we only use the user's launchpad username (assuming they have a full launchpad account). We don't have write access to this so we need to create our own username implementation - this should use the existing lp username if possible. We should probably keep track of which usernames have been used for openid logins so we don't allow them to be later picked up by another user if the first users changes their username. Users should be able to log in with their usernames in addition to their email addresses. Requires some more thought but I want to initially target this to the 2.7.0 release.

[Stuart Metcalfe (stuartmetcalfe)]

Re-assigning this to a safe distance past 10.10. It's too risky to land close to an Ubuntu release.

[Daniel Manrique (roadmr)]

SSO now allows managing the username directly, although Launchpad is used behind the scenes as storage. See commits 1435, 1438, 1467.