Registration - Allow email changes

[adam3smith]

Currently the email doubles as the username, which makes this ricky, but people change institutions (and thus emails) and I think it's in our interest to not have them have to create new accounts constantly.

@qqmyers would need to chime in with what's possible. I think the cleanest solution would be along these lines:

• create a username that's entirely separate from the email address, but create it automatically (e.g. as email name followed by a numeric suffix if needed
• allow login either with current email address or with username
• provide an interface to change email address (this should leave the username unchanged, though, of course).