Improve permissions for content box entities

[frega]

Status quo:

1. Currently in order to "view" the content_box entities embedded on a page, one has to have "View" access on the entity bundle.
2. In order to "add" content_box entities embedded on a page, one has to have "Add" access on the entity bundle
3. In order to "remove" content_box entities embedded on a page, one has to have "Delete" access on the entity bundle.

Problems arising:

1. As a consequence of 1. Anonymous can see any content_box by guessing the url.
2. As a consequence of 2. anyone can edit other user's

Todos:

• View (any) {Bundle} Entities in full view mode -> separate "View embedded entity" permission (view on node/123 if user can view node/123) from "View full entity" (ie. /content_box/dynamic_content/1234)
• View own { Bundle} Entities in full view mode -> scope this more tightly?
• Edit/Delete own {Bundle} Entities -> scope this more tightly?
• Provide a simple "content_box_access" hook analogous to "node_access" behavior?