Return avatar of user in REST API

Change-Id: I4839e6a830330f4ac8faa3ddf6b6e3039b5b409f
request #7303: Ability to easily reference a user (@mentions)

Change-Id: Id4e1d3124c786c9a126be58079af488ae7b89083
Merge commit 'refs/changes/48/2748/9' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  request #7303: Ability to easily reference a user (@mentions)
  Return avatar of user in REST API
This is Tuleap 7.3.99.28

Change-Id: I4cb7c01996de45e7e856ee9b5d4ee42758462cbd