Warn remixers when sampler is undefined

There are remixable images uploaded to the gallery whose sampler was not recognized so the sampler dropdown defaults to undefined, which still permits a remix’s Generate button to submit the job – but it immediately yields no images, showing no errors explaining the problem to the user.

Suggestions:

• Remove undefined as a selectable choice (i.e. put the disabled attribute on it) and treat the sampler dropdown as a required input field

• At Generate, pop a modal explaining the problem and containing a duplicate sampler dropdown to continue with a cancel button as alternative, but in any use case never permit the job to be submitted without a valid sampler

• Make the remixer smart enough to recognize unsupported samplers and ask the user once they click Remix if they’d like to use the closest one at Civitai that it knows about