tasks #7141 BUG: IE9 cannot deal with 4095+ css rules in one file

This tasks is a part of story #6003 be able to change the Experimental theme color

 * Split style.less into several less files (one per color)
 * Remove preview for IE

Change-Id: I03f172676f50b774523a543f9be81c07a39f1dc2
Merge commit 'refs/changes/11/2711/4' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  tasks #7141 BUG: IE9 cannot deal with 4095+ css rules in one file
This is Tuleap 7.2.99.14

Change-Id: Ia7464a911b083ee74803030d53fd362a6f72514e