rest: store archive_policy and export entities as resources

This allow to (only) retrieve the entities as resources on their
/resource/entity endpoint. We disallow any manipulation (POST/PATCH) on
this endpoint as we don't want this endpoint to allow changes (yet?).

Change-Id: I1260ec4f78d383e5bea25ecdbf08e44266cf005f
Merge "rest: store archive_policy and export entities as resources"