Add a test confirming that invalid moves should not be accepted (model-level validation)

Refs #480.
TreeEditor: Run the save() method of both instances involved in a node move
Remove an obsolete comment
Remove PageAdmin.unknown_fields

Refs #482.
Deprecate the automatic collection of model fields
Port symlinks and site extensions to feincms.extensions
Convert all bundled extensions to feincms.extensions
Add a snippet regarding singleton templates to the release notes