A new validate check before create or update

[nnseva]

Existent checks for create and update doesn't concern to the new attribute values.

It may be important especially for the foreign key values when the only selected person can link the created or updated instance to the object which is visible, but readonly for others.

It is proposed to create a check like verify_appendable() and verify_updateable() with parameters:

• model to be verified
• instance (?) to be verified (for updateable)
• dictionary of new attribute values