[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.