Add Wagtail setting to skip vendor contract upload

Hypha tracks whether a contract has been signed and countersigned depending on which role uploaded it and the context of the upload. This change creates a new Wagtail setting called upload_countersigned_contract. When set to True, whenever a contract upload occurs, the UI shows that the document needs to be countersigned and the document gets marked as countersigned. The net effect is that there is no need for two contract uploads, only one. When combined with the staff_upload_contract set to True, staff can upload a single contract document without other users' actions.

Issue ots/clients/ardc/hypha-tracker#219: Permit the Staff/Staff Admin roles to upload contracts.