Move all form validation to the backend

[dhess]

Because it creates better UX, we do some client-side form validation, such as stripping blank space off definition names in the CreateDefModal form. However, in an upcoming PR, we'll also check for duplicate names in the typedef and definition namespaces, again because this provides better UX. This is a rare occasion where the frontend makes assumptions about the language's semantics, which is something we try to avoid, in general.

We should fix this, either by not dismissing the form until the backend has returned a 2xx status code upon submission, or possibly via some sort of websocket validation mechanism.