Paginate rule groups, add infinite scroll to rules within groups

This addresses extreme slowness when you have thousands of rules in
potentially hundreds of rule groups. It can still be a bit slow even with
pagination and infinite scroll for very large use cases, but it's much
better already than before.

Fixes https://github.com/prometheus/prometheus/issues/15551



Signed-off-by: Julius Volz <julius.volz@gmail.com>