Add GET, OPTIONS route for milestones/:id/content

Change-Id: Ic4c26873a705aa2e4d57cf396ae01269223c5176
Merge remote-tracking branch 'gerrit/5584_milestones_content' into HEAD

* gerrit/5584_milestones_content:
  Add GET, OPTIONS route for milestones/:id/content

Conflicts:
	tests/lib/rest/TestDataBuilder.php
	tests/lib/rest/init_data.php

Change-Id: Ieb25eb84a97b5338929ed2e0a69b7f1831facb60
This is Tuleap 6.7.99.13

Change-Id: I4b08bb33df55981d483bf5b81cbbc5286db5cc3c