Do not check perms and required while importing data from the past

/!\ Big refactoring on a sensitive path: check of permissions. /!\

Move artifact creation and update in dedicated objects. This enhances
readability and enforces SRP.

Use of Template Method to import or create changesets.

Now "save new changeset" does not re-recheck isValid since it is already
done via another runtime path.

Change-Id: I8bb26d99433734cff1b8bb8327f9a193e647ae46
Merge commit 'refs/changes/04/1904/10' of ssh://gerrit.tuleap.net:29418/tuleap into tuleap-stable-master

* ssh://gerrit.tuleap.net:29418/tuleap:
  Do not check perms and required while importing data from the past

Change-Id: I2991c16ac3d810bc5adefef7ca705bf5142d3c30
Tuleap 6.10.99.25

Change-Id: Iaf60cef5a0041edcd2b0ff894a65e0303aa48440