Add PUT route for milestones/:id/content

Change-Id: I5f3c3e82934095f18712e01f7a1ff016500f0a86
Add new verification for REST Body content

* Ids are not passed several times

Change-Id: I50e033755c7d741e3f9cc2522b3aea9699acf722
Refactor BacklogItem related classes for better responsibility

- rename BacklogRow* into BacklogItem*
- introduce IBacklogItem interface to not have to deal with presenter in our logic

Change-Id: I11e768171c407992dc4cd810aebca5451ec20e13
Merge remote-tracking branch 'gerrit/5584_milestones_content' into HEAD

* gerrit/5584_milestones_content:
  Refactor BacklogItem related classes for better responsibility
  Add new verification for REST Body content
  Add PUT route for milestones/:id/content

Change-Id: I3b29cc3139a3a51f311598441f9d6869697943cd
This is Tuleap 6.7.99.20

Change-Id: I2ebb003b6925f6b24142d3148db9144af5577861