request #7347: Tv3 export to Tv5: migration is to restrictive, should not stop creating when something cannot be imported

Now, the import will try to:
* Create a fake initial changeset if the first abort
* Continue to create changesets if one abort (the missing one is lost)
* Create changeset with all possible valid data (if there are invalid data, they
  are skipped)

Change-Id: I3de9889b4e3d4c1ec4a8f6241b1441ff893bd5a2
Merge commit 'refs/changes/01/2901/3' of ssh://gerrit.tuleap.net:29418/tuleap into HEAD

* ssh://gerrit.tuleap.net:29418/tuleap:
  request #7347: Tv3 export to Tv5: migration is to restrictive, should not stop creating when something cannot be imported

Change-Id: I8ba097cdf294a43a08755c34eaec2225d9b749e4
This is Tuleap 7.3.99.64

Change-Id: I3be8b8b8c1837bbcce0adfb36eb3afc564a70c10