Requests for comment/Login via e-mail address

MediaWiki should allow login via e-mail address.

Background
Login via e-mail address is crazy popular on the Web. It's easier for users to remember an e-mail address over a site-specific username.

Considerations

 * Currently e-mail addresses are case-sensitive.
 * RFC 5321 says Bob@example.com and bob@example.com could theoretically be different e-mail addresses, but in practice that's crazy.
 * Write a maintenance script to normalize e-mail addresses to all lowercase in the database or just check at the application level?
 * E-mail address is considered private information.
 * E-mail addresses do not need to be unique (many accounts with a shared e-mail address happens).
 * Checking passwords is necessarily computationally expensive, so timed attacks are a real concern.
 * There is also CentralAuth to deal with, maybe.
 * Special:PasswordReset already has some of the same functionality/logic we're going to need here.

Workflow

 * Special:UserLogin is changed to accept e-mail address or username.
 * Always try as username first (there are usernames that are e-mail addresses...).
 * Alternately, force-rename users with "@" in their name. "@" is a reserved character according to $wgInvalidUsernameCharacters, so any usernames containing "@" are legacy names.
 * If e-mail address is authenticated, check password of account.
 * If the e-mail address is not authenticated, check password but discard the result to avoid timing attacks. Daniel Friesen (Dantman) (talk) 06:24, 30 January 2015 (UTC)
 * We want to avoid timed attacks that can determine whether an e-mail address is in the database.
 * Perhaps only check one account total?
 * This approach avoids needing to insert an intermediate screen for disambiguation.
 * This would mean that for e-mail addresses that correspond to multiple usernames, you would need to error and tell the user to maybe try an account name instead? (The error message presumably cannot give away that there were e-mail addresses that matched in the database, unless the password also matches.)