Prevent downgrade permission more than parent's

[pyphilia]

In membership table, we can change the access permission of a member on this item B in parent A. Let's say we have memberships: imA.permission = write and imB = admin for a member, updating imB shouldn't be less than write.

It's not possible to prevent this behavior in the frontend (unless we specifically fetch parent's memberships) since the received memberships are "squashed" (eg. we receive only imB because it's higher than imA).