Introduces MailGatewayRecipient management

Basically it does nothing (only added files for a better future).
- builds a recipient from a user/artifact
- builds a recipient from an email (and check the hash)

The mail gateway recipient format has the following pattern:
   %artifact_id%-%hash%-%user_id%@host.example.com

Exemple:
   101-5a2a341193b34695885091bbf5f75d68-123@tuleap.example.com

Which means that the user #123 will do some actions (e.g: add comment) on
artifact #101 (if the hash is valid)

Change-Id: Ifdea18820849d09350d027d673a9d2a18562b2b4
Merge commit 'refs/changes/37/837/3' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  Introduces MailGatewayRecipient management

Change-Id: I1d65c90e7bf1210ba445d2b3dbe1f8a6d8a3c0f8
This is Tuleap 6.4.99.3

Change-Id: I099be20595a692379f2162eb95bb84dadd86dd33