Move IncomingEmail and ServiceDeskEmail to Email namespace

We should move these classes under Email namespace. We'll need to ensure any scripts/tasks/workers that reference them (as part of our email flow) are also updated.

The following discussion from !102003 (merged) should be addressed:

• @felipe_artur started a discussion: (+2 comments)

  I wonder if this should be moved into lib/gitlab/email folder and defined as Gitlab::Email::Incoming, and also ServiceDeskEmail -> Gitlab::Email::ServiceDesk.

  Could be done in a follow-up if you think it makes sense.