tasks #7123: Select theme variant in user preferences

This is a task from story #6003: be able to change the Experimental
theme color:

* User can choose her variant in user preference page
* Variant is stored in User Preferences
* Variant is previsualized on the fly if user is currently using the theme

Change-Id: I25337bdaeba5f898c051ceda7f7554fcb624de2e
Merge commit 'refs/changes/99/2699/9' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  tasks #7123: Select theme variant in user preferences
This is Tuleap 7.2.99.10

Change-Id: I55d76b909bc951b4deab0519f521fd3971346cfd