tasks #6651 - Change style of add/remove column

+ Refactor TrackerReports.js to share dropdown management between add/remove
columns and add/remove criterion

Change-Id: Ieaca8c797e80fa9f5ce5e51baa1a6ed66c2847bf
Merge commit 'refs/changes/73/2273/2' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  tasks #6651 - Change style of add/remove column
This is Tuleap 6.12.99.37

Change-Id: I97c7e6334b949d485fec394b25f36901794a3cb5