This commit remove required alias on res.users model. It also removes the
inheritance of mail.alias.mixin. The field alias_id is kept and allow people
to use aliases on users if they want. However there is no default void alias
created for each user anymore.
Default from of mail.message does not use user alias anymore. Indeed this
creates issues with aliases not being correctly configured and can be
confusing compared to private channels. Using user aliases is therefore
now done manually instead of being a default behavior.
Thibault Delavallée