the html now contains a `data-prev-state` attribute which contains the
previous state, taken from the `operation` value in the form data (from
the submit button). This is used to seed the `currentState` of the
templateFolderForm. If not specified (or 'unknown', because the user
hit enter last time round), then set it to nothingSelectedButtons.
This will stop us repeatedly forgetting to add `novalidate` and
`autocomplete='off'` to our forms (which is how most of them are set
up).
It uses sensible defaults, based on how we most-commonly configure
forms:
- most of our forms are `post`ed (but this can be overridden)
- `autocomplete` should only be enabled where it makes sense, otherwise
it’s more annoying than useful (but this can be overriden)
- we should never be using HTML5 form validation because our own error
styles and messages are better
