story 2211 start date and end date

removing all requires for the one of common.php

replacing requires with common.php

now Milestone Factory fills milestone with its start_date if any

Start date of a current milestone is now displayed if any

moving getLastValue method to its parent

now display start and end date

* some refactoring about timestamp handling
* now showing the two dates or nothing
* modifying getLastValue

styling

now checking perms for start_date

Revert "moving getLastValue method to its parent"

This reverts commit f538a70a233bb94a5782fb927c4a275b0f412f71.

Conflicts:

	plugins/tracker/include/Tracker/FormElement/Tracker_FormElement_Field.class.php

taking account fo code review comments

taking account of the code review comments

Change-Id: I97c377ae4d4c9fc6e7e193f4ff6187a16c643e24
Merge commit 'refs/changes/58/258/3' of http://gerrit.tuleap.net/tuleap into stable

* http://gerrit.tuleap.net/tuleap:
  story 2211 start date and end date
This is tuleap 5.10.99.12