changeset status: set status to rejected if at least one reviewer rejected

Currently, a reject of a change by a reviewer does not affect the overall
status at all. A logical policy is to reject the change if at least one
reviewer rejects it.

Note: different repositories/organizations may require different policies
for the overall status. Currently, the policies for approve/reject are
fixed. A possible improvement is to provide several alternative
policies and allow selecting the policy per instance or per repo.