Added a NextJS frontend for training GPT on wikitext and executing some web text dataset related tests

[peacefulotter]

This PR adds another frontend to Disco, this time written in NextJS. The reason is simply because I do not know Vue at all while I am quite familiar with NextJS. In any case, I believe the Vue frontend should in turn entirely move to NextJS as it appears to be much better (and modern) in general, but this is more of a personal opinion.