Fix manager types scope (#991)

* Fix manager types scope

* Restore incremental mode and mention in developer docs

* Separate dev mypy config and regular one

* Document config files usage

mypy_django_plugin/lib/helpers.py

@@ -369,11 +369,10 @@ def bind_or_analyze_type(t: MypyType, api: SemanticAnalyzer, module_name: Option
     That should hopefully give a bound type."""
     if isinstance(t, UnboundType) and module_name is not None:
         node = api.lookup_fully_qualified_or_none(module_name + "." + t.name)
-        if node is None:
-            return None
-        return node.type
-    else:
-        return api.anal_type(t)
+        if node is not None and node.type is not None:
+            return node.type
+
+    return api.anal_type(t)
 
 
 def copy_method_to_another_class(