Update factories and form tests

Make LendFactory abstract and create two concrete subclasses: PastLendFactory
and FutureLendFactory. LendFactory provides values for 'user_id' and 'item_id'
fields, and the subsclasses provide a value for 'out' and 'due_out',
respectively. This change makes it easier to test the validation rules in class
``LendForm``, which say that a ``Lend`` object must have its user_id, item_id
and (out OR due_out) attributes populated.

Use these new factories in test_forms.py.