Submit buttons in the forms should be customizable, too

At the bottom of the registration form and the evaluation form, there is an `` with no (templated) `value`. As a result, the web browser sets whatever default synonym for "submit" in the system language, e.g. "Verstuur" or "Verzenden" for Dutch. If the survey is in a different language than the OS, this looks odd. In order to solve, add another field to the ButtonSet.

Inspect the registration and evaluation form templates and the ButtonSet definition mentioned in the issue. Confirm how submit inputs are rendered and how other button labels are supplied. Done means both forms use a customizable, survey-language-aware submit label instead of the browser or operating system default.

Written by the indexing model from the issue text.