Fix page 16 validation

Previously saved values were not retained on subsequent validation errors.  Validation messages were not being displayed (no top banner or inline message).
Fix page 20 validation

Previously saved values were not retained on subsequent validation errors.
Fix page 21 validation

Previously saved values were not retained on subsequent validation errors. Validation messages were not being shown.
Merge pull request #168 from alphagov/kk_validation_bugfixes

Textbox-list-item validation bugfixes