Extract url-generation to actor classes.
Add unit-test for label generation and refactor label generation.
Add null-actor implementation.
Move MEMORY_DB_LAYER to opengever.testing.
Replace `readable_author` with `Actor` lookup.
Use actor-lookup for document edit view.
Use sharing in browser overview sharing.
Move docstring to module level.

It describes how to use the module, not only how
the lookup works.
Let NullActor return emtpy string for label instead of None.

This preserves the behavior of `IContactInformation.describe`.
Add classmethdos to create specific actors.
Use `Actor.lookup` instead of `ContactInfo.describe`.
Opentaskreport: Use `Actor.lookup` instead of `ContactInfo.describe`.
Fix me: Fixed UserActor, the label function hasn't worked.
LaTex tasklistings: reworked tasklistings.

Some of the changes are only temporary and will be removed after implementing
the issue https://github.com/4teamwork/opengever.core/issues/289.
DossierContainer: Added `responsible_label` property.
Droped no longer used `IContactInformation.list_inboxes`.
Reworked ListGroupMembers view, use the ogds_service and Actor class.

Test is missing for the ListGroupMembers view.
DocumentOverview: use Actor.get_link instead of `IContactInformation.render_link`.
CheckedOutViewlet: use Actor.get_link instead of `IContactInformation.render_link`.

Added tests for the viewlet.
DossierByline: use Actor.get_link instead of `IContactInformation.render_link`.