[Feature Request] Force Write-once / Non-updatable Updates for some users

It would be nice to support in `content.json` a way to specify that some IDs are allowed to send updates / write files to that location but only once, i.e. peers should ignore future updates of that files coming from said IDs.

This could be useful:
- in a write-once site, where users cannot censor themselves.
- in zerowiki, where small updates could be saved as diff wrt others, without having to trust the original user to not delete its edit.
- Implementing it in the syntax of content.json could also restrict the admin to change some part of a site after they have been written (also by having the content.json itself be subject to such a policy) which in turn could allow to create non-censorable sites (also by the owner himself) that would act as append-only logs.

In particular the last use could at least in theory solve #1288 and could partially solve some need for the more difficult site archival feature, since this one seems to be only a couple of line change.

Start with the content.json format and the behavior described for peer updates, then inspect how update permissions and file writes are represented. Compare the proposed policy with issue #1288; done means specified IDs can write a location once and peers ignore later updates from them.

Written by the indexing model from the issue text.