grist-core has path to server-side requests via websocket

grist-core is a spreadsheet hosting server. Prior to version 1.7.7, a user with access to any document on a Grist installation can use a feature for fetching from a URL that is executed on the server. The privileged network access of server-side requests could offer opportunities for attack escalation. This issue is fixed in version 1.7.7. The mitigation was to use the proxy for untrusted fetches intended for such purposes. As a workaround, avoid making http/https endpoints available to an instance running Grist that expose credentials or operate without credentials.