Category revamp

(Only two top commit are relevant, the rest comes from other MRs.)

This revamps the category section, making it a bit more modern, modernizes the code with react hooks, simplifies a lot of patterns, etc. Functionally equivalent, though. I think this should serve as an example of where the web app is headed to; in the future I'd like that we'd do more refactorings like this one.

Not quite satisfied with the buttons to edit/remove categories; would be nice to have something thinner, maybe.

Also fixed the rc-color-picker used when there's no native input type=color. The lack of complaints about this color picker not working is either a good sign that we can remove the fallback, or a bad sign that we're not getting enough user feedback :-)

Fixes #622 (closed).