Update converters to take a consistent set of parameters.

As suggested by Anssi. This has the slightly strange side effect of
passing the expression to Expression.convert_value has the expression
passed back to it, but it allows more complex patterns of expressions.

docs/howto/custom-model-fields.txt

@@ -477,7 +477,7 @@ instances::
     class HandField(models.Field):
         # ...
 
-        def from_db_value(self, value, connection, context):
+        def from_db_value(self, value, expression, connection, context):
             if value is None:
                 return value
             return parse_hand(value)

docs/ref/models/expressions.txt

@@ -412,7 +412,7 @@ calling the appropriate methods on the wrapped expression.
               clone.expression = self.expression.relabeled_clone(change_map)
               return clone
 
-    .. method:: convert_value(self, value, connection, context)
+    .. method:: convert_value(self, value, expression, connection, context)
 
         A hook allowing the expression to coerce ``value`` into a more
         appropriate type.

docs/ref/models/fields.txt

@@ -1630,7 +1630,7 @@ Field API reference
 
     When loading data, :meth:`from_db_value` is used:
 
-    .. method:: from_db_value(value, connection, context)
+    .. method:: from_db_value(value, expression, connection, context)
 
         .. versionadded:: 1.8