Add write permission to objects only if necessary

[leplatrem]

Currently, when an object is edited, the write permission is given to the creator/editor.

This way, there's always someone with write permission.

Problem, revoking permission on children is hard (#144).

One idea could be to add the write permission only if the user has no write permission on the parent.

Examples:

• write on bucket → create sub collection → no write on sub collection
• write on bucket → update sub collection → no write on sub collection
• read and collection:create on bucket → create collection → write added on collection
• read and record:create on collection → create record → write added on record

Permissions specified via settings should be taken into account with this approach. Related #892

Careful with was done in #837