Merge client-side and server-side form validation

The new experiment and optimisation pages currently use submit and on-change validation respectively. In addition there is validation code server-side (which is necessary for an API). To abide by the DRY principle, perhaps the server-side code can be sent as a function to the client-side, and both forms can utilise on-change validation.

Start by locating the experiment and optimisation pages, their form validation entry points, and the existing server-side validation code. Compare the submit and on-change paths, then determine whether both forms can share validation while preserving the server-side API checks; done means the two forms use consistent validation without duplicating rules.

Written by the indexing model from the issue text.