Only users from administrators group should be able to create report models

For security reasons, we should allow only people from administrators group to be able to create templates of BIRT reports.

Ideally this should be done in Admin Center, in a "Reporting" section, but, as a first step, we could display " create a Birt Report Model" button only in the userworkspace of people being part of "Administrators" group, and display the "Manage" tab so that the visibility of the document can be changed