imap: Added initial support for METADATA extension.

imap: Added initial support for METADATA extension.
For now this is enabled only when imap_metadata=yes setting is used. The
setting will go away once the feature is complete. Also mail_attribute_dict
must be set.

TODO:
 - Metadata doesn't work for public namespaces. There should probably be a
   mail_attribute_public_dict setting for that.
 - There isn't any kind of quota or other limits
 - After ENABLE METADATA start sending untagged METADATA entries to clients
 - /shared/admin should probably return postmaster_address URL
 - Check if we handle ACLs correctly
 - RFC says that it SHOULD be possible to set METADATA entries to \NoSelect
   mailboxes. We probably will never allow this though.