[RCD] Remove user.alias column

That column is used only for log in. The current behaviour allows only one alias. Also not every installation is using it at all. What I propose is to:[[BR]]

1. remove this column.
2. add an option which will search for the username in identities table.

SELECT u.* FROM users.u JOIN identities i ON (i.user_id = u.user_id) WHERE email = ? LIMIT 1

So, this way searching for alias would be optional (which will improve performance, when not needed, e.g. there's a plugin which do alias mapping) and will give more possibilities.

What I'm afraid of is that there's a use-case where alias is not an email address.

http://trac.roundcube.net/ticket/1488581