Handle mail reception (tasks #6950 + tasks #6951)

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

In this changeset, we read the incoming mail, parse it and inject the
first body we find as a new follow-up comment.

Change-Id: I02f2a7ff16c9db3e6c600acd2d3d18f955518856
tasks #6989: Better handling of various email format

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

Chosen strategy: Use the first text/plain alternative we can find

Change-Id: Ie7ba1bcc2d775a5c764826bb74899455efa301f7
Merge commit 'refs/changes/83/2583/4' of ssh://gerrit.tuleap.net:29418/tuleap into HEAD

* ssh://gerrit.tuleap.net:29418/tuleap:
  tasks #6989: Better handling of various email format
  Handle mail reception (tasks #6950 + tasks #6951)
This is Tuleap 7.1.99.13

Change-Id: I26980ec3ecab68b466ca8683f12d00e7f6c04419