- PermissionSet reads the stored JSON tolerantly and writes an object
of integers; FrontendPermission is the catalogue row
- MergedPermissions, HasPermission and UserHasPermission port PHP
getMergedPermissions and Winter's hasPermission: groups merge with
most positive wins, the user's own values override, held means
exactly 1, wildcards as in Winter
- User gains Permissions (never serialized) and declares the avatar
attachment relation the user API already writes
