Use rendition variants to isolate non-versionable stored renditions

Currently the per-user renditions are using ACLs to isolate renditions. This is not ideal because:

• it makes the documents visible instead of being an implementation detail of the rendition service,
• it makes an Administrator use the rendition from a random user if one exists.

Instead of ACLs, use an explicit field (renditionVariant) in the rendition to discriminate. For per-user rendition we configure the rendition provider with <variantPolicy>user</variantPolicy> and then the rendition variant is computed as user:bob or administratoruser: in case the user is an Administrator.

A StoredRenditionManager interface is also introduced in order to configure how to create (createStoredRendition) and find (findStoredRendition) a stored rendition in the platform if the default is not enough. It's used with the following extension point:

  <extension target="org.nuxeo.ecm.platform.rendition.service.RenditionService" point="storedRenditionManagers">
    <storedRenditionManager class="com.example.MyStoredRenditionManager" />
  </extension>