Write scopes do not grant delete permissions

Following on from previous discussions on what is meant by "write permissions", there's a clarification that these should not include delete permissions, which should be separated into a new scope. Possibly we should rename the WriteXYZ scopes also for clarity.

This will become relevant when we implement user-to-user permissions (projets/projets-clients/open-food-network/data-permissioning-module#56 (moved)).