Use dialog element to manage focus inside modals

[sivakusayan]

This should close #89. We're going to rely on the browsers' implementation of the <dialog> element instead of trying to do our own focus handling. There is still the issue of the dialog's close "X" button not having a focus outline, but that's pre-existing and can perhaps be resolved in a separate PR (mostly because I'm not sure if ya'll have any strong opinions on what the focus outline should look like).

As an aside, this is the first time I'm writing any Clojure code at all that isn't just copy-pasting code into a REPL from some book, so don't be shy about feedback 🙂

[borkdude]

@oxalorg ping