Only allow admins to create sourceaccess/access repositories flags

Webui::RepositoriesController.create_flag allowed creating the
access/sourceaccess flags, while in
SourceController#project_command_set_flag this is only allowed for
admins.

This was fixed in master in:
https://github.com/openSUSE/open-build-service/pull/7169/commits/5f89387c86be9bcefffe64e78125efa88e3c3c4f