Refactoring: Enforce Single Responsibility Principle

MailGatewayRecipientFactory should not returns anything other than a
MailGatewayRecipient object.

This is part of story #6301: reply to a follow-up comment by email

Change-Id: I7c8c831017b8eec10858f1727991fdbd2bbc39f3
Refactoring: Proper namespace.

All should live in Tracker\Artifact\MailGateway\ namespace.

This is part of story #6301: reply to a follow-up comment by email

Change-Id: I171e9d26b3bef1993ac628a557d679c97a0a45b7
Merge commit 'refs/changes/94/2594/1' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  Refactoring: Proper namespace.
  Refactoring: Enforce Single Responsibility Principle
This is Tuleap 7.1.99.16

Change-Id: I78bee3b183bd29945606dd568c7a85346083da70