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
Merge remote-tracking branch 'stable/master' into 2211_start_end_date_on_release
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
fixing display bug
Merge remote-tracking branch 'origin/2211_start_end_date_on_release' into tuleap-stable-master

* origin/2211_start_end_date_on_release:
  fixing display bug
  taking account of the code review comments
  taking account fo code review comments
  Revert "moving getLastValue method to its parent"
  now checking perms for start_date
  styling
  now display start and end date
  moving getLastValue method to its parent
  Start date of a current milestone is now displayed if any
  now Milestone Factory fills milestone with its start_date if any
  replacing requires with common.php
  removing all requires for the one of common.php
Tuleap 5.10.99.18