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.
This will download the missing messages into a temporary directory and
decrypt them along with the metadata so we can attempt analyze them once
submitted to see what is going wrong.
Add debug commands to CLI to diagnose potential bride problems.
Currently we only have a command which validates whether the state of
all the mailboxes reported by IMAP matches what is currently available
on the proton servers.
