Form validation framework

Currently form validation on the various pages is done in an ad hoc manner and there is lots of duplication. The logic related a single field is spread to at least 3 or 4 places. There is need for a form validation framework such as in @luontola's previous project.

Start by surveying the ad hoc form-validation logic across the various pages and compare it with the form validation framework from @luontola's previous project. Done means the duplicated logic for individual fields is replaced by a shared framework, but the issue does not define its API or scope.

Written by the indexing model from the issue text.