Textbox-list-item validation bugfixes

Fixes this bug:
https://www.agileplannerapp.com/boards/187120/cards/7993

It was a wider issue for other pages using the textbox-list-item form fields that no-one had noticed yet, so I fixed pages 16 and 21 too, where old values were not displayed on subsequent validation errors, and validation messages were not displayed (inline or at the top of the page).