Handle removed portlet types

Problem/Motivation

Homebox breaks (shows an error) if a HomeboxPortletType is no more existing.

We should handle this better, not yet clear how to do it best.

Steps to reproduce

1. Enable the Block Portlets submodule
2. Place a Block Portlet in your homebox
3. Uninstall the Block Portlets submodule
4. See your Homebox broken

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes