Respond to incomplete URLs with an HTML form for composing a complete URL

[pictographer]

We have enough information in api.toml to automatically generate a custom data entry form for each URL. We could help the user by instead of merely describing their errors, presenting them with a form that enables them to compose correct URLs.

For example, suppose a route requires an Integer and a TaggedBase64 value. The form could have text fields for both and an output field that combines the user's inputs with the static portion of the URL for the user to copy as well as a link to try the URL.