Support to enable/disable the Jira integration in the UI

On the ProjectInfoScreen there is now a dropdown list for the
plugin.its-jira.enabled parameter which offers the values true,
false, enforced and INHERIT. Project owners can change this parameter
and save it. If the Jira integration is enforced by a parent project
the dropdown list is disabled.

Change-Id: I440b0fcee112bf4d0dea9339e6080c1fcd9a23b7
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>