Provide catchall_* methods to the Handler class

First version of a CatchallAlias class I based the CatchallAlias class in catchall.py heavily on the Alias class, but by copy, not deriving. The two are functionally related, but the implementations are too different because CatchallAliases have no localpart.

Add database table for catchall destinations Catch-all aliases are stored for a domain. Since there can be multiple destinations per domain, this is done using a 1:n relation on the gid. This commit extends the SQL schema definition and also provides appropriate upgrade DDL.