Added
- The `validation_error()` function.
- The `num_pages()` method on `DAFile`, `DAFileList`, and
`DAFileCollection` objects.
Changed
- On the mobile `signature` page, the "Clear" button is replaced with
a "Back" button.
- Python `validate` code now works on document uploads.
Fixed
- The `question back button` was missing from the buttons on the
desktop `signature` page.
- Problem with spacing of the `question back button` next to other
buttons.