Add new type Inbox Container

Added new type InboxContainer. Which is used in setup's with multiple `OrgUnit's` per `AdminUnit`, to store all inboxes (each for every `OrgUnit`).

This way of implementations is the better alternative, than the previous implementation with a `Maininbox` containing all `Subinboxes`. Because an specific container type brings the following benefits:
 * It's not possible to create content on the `InboxContainer` respectively `MainInbox`.
 * The permissions are much better understandable.
 * A user wich has access to an inbox, but not on the inbox of the current `OrgUnit`, can be informed with an status message.


@lukasgraf @deiferni please have a look.