Feat: convert Popup component to use new <dialog> HTML element

[franknoirot]

The newly-stable <dialog> element has a number of good features such as keyboard capture, support for deep nesting, and simple backdrop styling. After "one-for-one" migration to NextJS is complete, we should adopt this new dialog and simplify our Popup component considerably. It will also allow components that have associated popups to keep the popup dialogs next to them, instead of needing to be at the page level (at the bottom of the DOM no less), making understanding the codebase that much easier.