Rather than accessing the Bridge user list, each user register their
individual SMTP service with the server manager.
Note that some dependencies on the user data are hidden behind the
`UserInterface`. These will be removed in a future patch.
Refactor code to isolate the SMTP functionality in a dedicated SMTP
service for each user as discussed in the Bridge Service Architecture
RFC.
Some shared types have been moved from `user` to `usertypes` so that
they can be shared with Service and User Code.
Finally due to lack of recursive imports, the user data SMTP needs
access to is hidden behind an interface until the User Identity service
is implemented.
